OK, this is a odd issue to say none the less. I have a build in mind for a full size computer that i can run programs for Amateur Radio and other application and possible a portable server. But here's the catch I want to run the whole thing with a 12 V DC power in. now I not sure of the specks just quite yet bet i need a way to make the power out to the components at the correct voltages and the proper pin outs.
Now this can be solved with an inverter and a "off the shelf" Power supply but for me i find that to be the lest efficient way to do solve the problem and I don't what to loses any power due to phantom loads. also a laptop just dosen't have the horsepower to do what I'm planning on doing. plus i think it would be a interesting challenge.
You should also be able to find a 12V DC to DC supply. Computers in cars was quite common, so supplies like this exist, they wont be hardcore gamer type stuff however - that's not something you'd run off a 12V battery.
etc etc, not recommending that product because I'd have no idea, just an example.
You could use a thin mini-itx motherboard which is directly powered by DC, such as this:
It uses socket 1150 and should work with most Haswell CPUs and supports DDR3 1600 SO-DIMMs. it can also power SATA devices directly from the motherboard and can also use mSATA. Using this removes the need to get a DC power converter but obviously has it's limitations such as not using able to use most PCIe devices like graphics card and such.