Plenty of off the shelf options out there... Popcorn hour, WDTV, Boxee, AppleTV, Sumo-Vision to name a few.
Same as there are plenty of PC based applications... XBMC, Windows Media Centre (good if you add Media Browser + some codecs), Boxee, Media Portal, Tonky to name a few.
Guess the best thing is to try a few and decide what you want. I've played with WDTV (including custom firmware), Tonky + PS3, Media Portal, Windows Media Centre + Media Browser, Boxee PC and XBMC - only XBMC did everything I wanted and looked how I wanted