From Lifehacker:
Windows/Mac/Linux: Temporarily turn your computer into a home theater with the GeeXboX live CD. All you need to do is drop the GeeXbox CD into your drive and restart. When you are done with GeeXbox, remove the CD, restart your computer and you will return to your existing operating system. Originally created just to play DivX videos, GeeXboX can now play virtually any file format you throw at it. GeeXbox has extremely low requirements and can be used to turn an old computer into a media center. Linux users who don’t have the libdvdcss2 codec installed can boot into GeeXboX and watch DVDs without any setup. GeeXboX is a free download for Windows, Mac and Linux.
Get GeeXboX here.
