Because if you didn't know about it before and you thought it was crappy and Microsoft, there's no helping you, Mr Preconceived Notions?
It's only just gone multi-plaform now, but it's always been open source (what with the slightly dubious legal status of compiled versions), with a wealth of codecs and stuff. Given the extreme lack of alternatives, Softmodded Xbox + big HD + XBMC = the best way to get a large-capacity media jukebox on the cheap!
There's been an OSX version for a while that works really well. All I want is for someone to make a Wii version.
I've got my Wii all homebrewed up, and MPlayerWii plays files from external USB disks... but has no interface. Gah.
What Kenny said, and I assume you've ignored any post/threads that involve XBMC because I've mentioned XBMC for Linux on here quite a few times!!
But anyway, download and have fun!
Full Release version has now been made available.
No longer in Beta. I'll try the Live version some point soon!