HD from Netflix, Amazon, YouTube, and Hulu primarily.
Hulu Plus requires a paid subscription ($8/mo) and I believe a lot of their content is now only available to subscribers. I don't recommend it though unless there are a lot of recent network shows you watch and you don't have cable, satellite or a good OTA set up. However they currently offer a free month trial if you have Windows 7.
Edit: On a related note I actually had trouble getting HD on some shows through the PS3's Hulu Plus app, shows that displayed fine on another TV attached to a Roku. If I fiddle around with the PS3 and TV settings, I may be able to figure out the problem, but probably won't bother since I won't continue with Hulu Plus once my free trial is over.