I bought it to play with GPIO, but the Pi sucks when using the Desktop. No 3D acceleration (yet) and you can't adjust the resolution without rebooting and I can't use the full rez of my monitor of which is suppose to be supported by the pi, (1920x1200) but it doesn't work right on my monitor.
I used mine as a Mumble server for awhile and it worked great. I setup an Apache server so I could relearn that process. Thinking about setting up an email server for the same reason.
My old pi was used for many different things. I plugged a battery pack into it and used it to create a time-lapse video of a football tailgate party. I had XBMC on it at one point as well as mame for arcade emulators. What I would really like to do is create a mini arcade machine using a pi. It would be complete with a joystick and 6+2 buttons. Maybe an 8-12 inch screen.
I always run lightweight versions of raspbian that don't have the gui so i'm forced to nav though the terminal through ssh as well. If I want something to work, I have to figure it out myself or research the solution. As i'm writing this, I'm wondering if I can setup my pi to compile C++ code using the GCC compiler over the network from my windows machine instead of doing it in a vm. hrmmm
It really is a great way to learn linux. If you also have an arduino you can interface them, then ssh into the pi to send signals to the arduino. There's just so many things you can do. I personally, have a bot I've been working on, that watches IRC and my e-mail and notifies me on certain messages from those, reminds me to take breaks, reads what's on my calendar for that day, and reads my twitter mentions. I'm currently adding a feature to look for job postings for me to apply to. If you're willing to learn some scripting the sky's the limit.