-Hacked it to enable SSH and read/write FS
-Installed Mplayer and XBMC
-Made it so a folder called ATV on my desktop computer automatically syncs with the ATV using rsync regularly so whatever I have downloaded is always on the ATV
-Ordered and installed a Broadcom CrystalHD mini PCI card that renders video and takes processing that away from the ATV's limited CPU
-Installed kexts that support the above and a nightly build of XBMC so I can now play 720 and 1080p media using XBMC
Works perfect for me. I could install Linux on it but both myself and my partner love Apple's movie rental system and the iTunes integration for our music. So by applying the above hacks we get everything we need.
It does also support network shares with a bit of hacking.