I'm surprised the PS3 hasn't been mentioned. I bought one mostly for Blu-Ray support, but the media streaming functions have really matured. It now has native playback of Divx and xvid as well as WMV and H.264. The quality is really good over HDMI and I have been using it quite a bit.

I understand that a PS3 is a lot more expensive than an AppleTV, but Blu-Ray support is nice to have. No hacking needed for divx is also a plus.