Yes it is possible, (Ubuntu is one example) but pointless. Why wouldn't you just buy a PC to do that? You can get a much more powerful PC without paying for the overhead of Apple's industrial design and pre-installed software.
When you buy a Mac, you're paying for Mac OS X, iLife, and Apple's chic design. None of that makes any sense for running a Linux server.
Also, Mac OS X makes an excellent unix-ish server for all those things.