Small form factor (SFF) PCs have been around for a while, but in the early days, you could only get them from a company willing to make a custom design. Now, thanks to a burgeoning SFF subculture, you can build one yourself.
🎥: Optimum Tech
Update (February 5th, 5:00PM ET): We recommended the NZXT H1 PC case, which has been recalled for safety problems (electrical shorts) with its PCIe riser. We have removed that card and updated this post accordingly.