And I'm pretty impressed. Okay, so it's a bit crashy, and it's a little hard to find, but it IS free, and while it works it works great. Get yourself one of the latest CVS builds and you'll be going off.
The nice thing about XBMC, as it is often called, is that it is expandable. You can use XML to customise it, and Python to program it (lucky that I have a book on Python already). This is great, as it means such things as TV guides can be added to it. Not to mention direct support (via MySQL and SAMBA) for my MythTV recordings.
PS. To mod the XBOX don't go past Softmod Installer Deluxe, which you can find here, but make sure you get the latest version 4.5, especially if you have a PAL XBOX (this caused me no end of confusion). And if you use Splinter Cell to softmod, make sure you get the matching (PAL/NTSC) game save!