Probably late to the game, but I just put Boxee on my dual core i5 box attached via HDMI to my HDTV. I now have zero use for an AppleTV, Roku, or whatever, because this does everything I need it to, in addition to having all the app stuff. Right now I'm running local files recorded with my Hauppage HD-PVR as one repository, and movie files encoded for iPad on a Linux server downstairs in the basement. It plays everything back smoothly, without a glitch. Also plays all my iTunes directories that I've got spread out throughout the house.
The nice thing is that they've got an iPhone/Android app that works as the remote, and it allows for total control over the app without a keyboard. It's also nice because I can control it, or my wife can use it when I'm at work with her iPhone; I don't need an additional device.
Check it out! I don't recall what everyone else is using, and I'll likely experiment if there's even better stuff out there, but at the moment this is way better than any of the dedicated boxes are, IMO.
DL'd this just because of you, works better than just running VLC on my desktop plugged into the TV.
Now If I could just get rid of the interference the HDMI from the PC causes on my cable picture.... I have to pull the cord out when I'm not using it. Maybe I should get one of those HDMI mutli-taps and isolate the cable.