Since there is no pubicity for free and open source games, maybe it's useful to not only list games, but to also give some general facts about FOSS gaming:
1. Everybody already know that it's mostly gratis, not that the "free" in "free and open source" means gratis, it means "free" as in "freedom", and that means i.a. that you can mod the game as much as you want and that the tools to mod the game are also free and gratis. A lot of FOSS games provide an easy programming language for mods, the engine is open source, so you can dig as deep as you want to mod the game, and some FOSS games feature "in game" editing, with a graphical editing interface that is accessible via the game menu.
2. I'm not going to list a bunch of titles here, because there are many titles in many different versions with different features and different mod compatibilities, and the version that is available often depends on the distro, even though it's always possible to install whatever, it might just not be as simple as just clicking it in the package manager, you might have to compile and/or build it yourself, which is not hard to do, but since it requires some particular measures (like making a different account on your machine, using a build environment and avoiding root access, that's not for this post).
3. All the servers of all the FOSS online games can be run without license cost. And running a FOSS game server is not as obnoxious as running a closed commercial game server, there are far less assholes and idiots on them, it's like stepping back in time pre-XboxLive. This has a lot to do with the fact that a lot of the exchanges between community members do not only relate to playing the game, but also to developing or modding the game. That automatically uplifts the community communications and the gaming experience. A user may be attracted to FOSS games and the community around them because they don't cost money and they need info on how to play gratis games, but stay because of the great community and the thrills of being able to realize their gaming dreams themselves with the help of the community. It is a huge misconception that one needs to be a programmer to fit in, it's not extraordinary for users without any knowledge of any open source platform or programming skills, to start making their own mods within two weeks after joining a FOSS gaming community online.
4. Hardware requirements are far less important on a GNU/Linux system with FOSS games. Some games can be harder on graphics, most are not. Important thing to know is that AMD graphics solutions work better with open source drivers than nvidia solutions, and that nvidia graphics solutions work better to a certain extent with proprietary drivers than AMD solutions, but you can't do as much with them as with AMD solutions with proprietary drivers. If you want to use open source drivers, the old nvidia 9800GT is momentarily the fastest nvidia card with open source nvidia drivers in linux, whereas all modern AMD solutions work fine with the open source AMD drivers. If you also use OpenCL acceleration for stuff, AMD is the best graphics solution by far in linux, nvidia is nothing but trouble, even when you install the proprietary drivers and libcuda packages, and nvidia succeeds in delivering a stable driver, AMD just works, nvidia just doesn't (for instance Darktable or other OpenCL enabled applications). If OpenCL acceleration is necessary, NEVER get an AMD card with less than 2 GB of GDDR RAM though, because only half will show up for openCL, and for good acceleration, you need about 500 MB of graphics memory to be reserved to openCL, which means that you'll need double that available on the graphics card, so that means that you should have a 2 GB RAM graphics card, but the amount of acceleration will be spectacular. But OpenCL acceleration is not required for gaming.
5. As far as the other hardware is concerned: SSD's are much more efficient in linux, not only because the filesystem is much better, resources management and use is much better, and you can even opt for Samsung's F2FS linux filesystem that is specially made for flash based storage solutions and is showing great promise in terms of performance, but also because a full linux install with a huge bunch of software, hardly ever takes more space than 15 GB, and you'll already have a tough time even getting to that much, a normal install with a good collection of software will certainly be under 5 GB, whereas Windows takes about 60-70 GB after a while, which means that just the operating system occupies the full efficient storage space in Windows on a 120 GB SSD, whereas even a cheaper 64 GB SSD would still be running efficiently in linux. So you can easily run your games from your SSD in linux, and still not go over 66-75% occupation so that the SSD stays at full performance. Games and levels in linux typically start up in a few tenths of a second to a few seconds, even on slower and older computers and from a HDD. Related to this is that downloading a gratis game from a repo only takes a few seconds to a few minutes at most, and you don't have to wait for payment processing, slow all-night downloads from Steam etc... you can start to play within seconds to minutes after deciding to want to play a game, and if you don't like it, you can just download another one, and you don't lose money.
6. Also concerning hardware, there is no use in investing more for "enthusiast" or "gaming" grade parts, because in linux, those show their true nature. Investing in a more expensive x87 platform instead of a cheaper B87 platform for instance, will reward you (also in Windows by the way, but I'm not even starting about that) with a 30% framerate LOSS. Yes, you heard it right, an expensive X87 board is between 10 and 30% slower than a cheaper B87 or H87 board in games. And in general, the framerate on a linux system running FOSS games will be exponentially higher than running games in Windows, so unless you also want to use the system for large parallel floating point intensive tasks like research or engineering or mathematics, there is no use paying more for less. That is also true for processors, in linux it become suddenly very important to not have to deal with closed source software problems, so you'll want to run MS-Windows in KVM or Xen, so that you don't lose performance in gaming on Windows when you feel like doing that, but you can also easily snapshot your system so that you don't have to deal with typical Windows breakage and problems and endless updates. For that, ironically, you need to buy a cheaper processor, either an AMD processor, or a cheaper fixed multiplier Intel one, with a cheaper board, either an AMD board, or an Intel Q87 or B87 based board that is compatible if you prefer a board from major manufacturers, or a board from an alternative manufacturer that offers cheaper boards, like AsRock, ECS, Zotac, etc..., which are not in cahoots with Microsoft, and don't block certain very important safety features. The general rule for the best free and open source software result for general and gaming use is: get the hardware that is sold the most, the chances that it will be optimized by the community are the best, so the performance will also be better.
7. Also concerning hardware, there is no use in getting more than 8 GB of RAM on a Linux gaming and general use system, in fact, there is no use in getting more than 4 GB of RAM. Typically, if your PC has 4 GB of RAM, half of that will be used as a RAMDisk at boot, because the system only needs 2 GB of RAM at most, and you'll see that the system will not even bother using any swapfile at that time. A linux system will typically start to use a swapfile when the system has less than 2 GB of RAM and is running a whole bunch of RAM heavy applications at the same time. The most RAM intensive applications in linux are typically Chromium and Firefox and Unity and Cinnamon, most FOSS games will run just fine with 384 MB of system RAM in linux and a RAM efficient DE.
8. Linux has a huge choice in emulators. These can be emulators, or virtual environments. Virtual environments run faster, but for most emulations/virtualizations of classic gaming consoles, that is not an issue, because they will always run fast on a linux system. The games themselves are often available for gratis download, however most of the emulator games do not come under a free and open source license, so check what license they're under before using them so that you know how you can use them legally. This is mostly well documented and its hard to do something wrong if you check the websites of the providers of those games. And as linux is compatible with almost every piece of hardware known to mankind that has a digital connector, most controllers and stuff will just work immediately and driverlessly, like most hardware does on linux, and will be fully configurable.
9. Some games will have an open source engine, but closed source content. This is normal and perfectly acceptable. Software should be open source, the work of artists should not, because that is an individual artistic expression that has a value of its own, and software is not, but should be for the benefit of mankind, as it is a technical design and not artistic in any way, and cannot have any value on its own, because it depends on a platform or another person's or group of persons' inventions to even function. That means that you might opt to buy gaming content or might be allowed to use gaming content for free, but with a limited license to personal use. Free and Open Source is about protection of rights, and that includes rights of artists. You can however also use free community content or your own content on those gaming engines, there is plenty of choice.
10. Not all linux distros are ideal for gaming. Some do not follow upstream developments as closely as others. The best gaming distros are arch linux based, because - in huge part thanks to the AUR - arch linux has the most recent games available prebuilt and prepackaged, and most hardware features will work because Arch is bleeding edge. Arch itself is the ultimate choice, but will require some nerves and some tinkering, and the easy alternative is Manjaro linux, which is not quite as bleeding edge as Arch, but still much more than Ubuntu or Mageia, and will offer an automatic configuration of graphics solutions with proprietary drivers and some other optimisations out of the box. Whatever anyone says, Ubuntu is not ideal for Open Source Games, it is focused on closed source commercial games, and even in that field, it's not the ideal distro, but again, Manjaro or Arch are. Fedora used to have a games spin, and is very good for gaming also, but doesn't follow the upstream game development as closely as arch, and therefore will not offer the latest and greatest. OpenSuSE is pretty much on par with arch as far as following upstream goes in games, but is not a bleeding edge distro, so not all the newest hardware features will always be available, and they won't have all the packages of all the games in their repos, but they do offer the very handy "Yast 1-click install" feature for proprietary graphics drivers and a lot of other software.
There's a lot more to be said about FOSS gaming, but I think that is a good start already. Anyone care to append?