Streaming doesn't work with iTunes player. It uses DLNA, which is supported by Windows Media Player 11 and some other programs (but not iTunes player, unfortunately).
There's the photo-slideshow stuff. It's really under appreciated, imo, since you can just plug in your digital camera via USB (or use the SD/CF/MMC slots) and have all your pictures on the big screen. The PS3 controller actually works pretty darn nice for browsing pictures, since you can zoom/rotate and change pictures really easily. Plus there are some nice "photo-album" effects if you want to setup a slideshow.
The browser isn't as full-features as a desktop one, but there's still some really useful things you can do with it. I personally have it setup for TVguide.com
, so I can check what programs are on without having to go to the computer. Not rocket science, but still very useful.
Zoom also works well for porn. I should have mentioned that before.