You can watch videos on YouTube, Vimeo, Apple Trailers, and WSJ Live, listen to internet radio and podcasts, check sports scores, browse Flicker, access your iTunes library, and view your Photostream(s).
"Like are there any apps besides the default ones?"
The primary reason I got an apple tv was so that I could view my photos on my TV without having to hook up my computer.
If you're on a mac OR a Windows PC, you can use HomeSharing to view your photos on your ATV via iPhoto, Aperture, or simply organizing them in folders on your computer. This is managed through iTunes.
Homesharing also lets you view any videos you have in iTunes. These can include DVDs you have ripped using Handbrake (free). Or home videos in your iPhoto library or that you've dragged into iTunes.
The hassle I save from not having to move/plug in my computer to my TV to watch this stuff is well worth the price of the Apple TV.
I rarely rent from iTunes. I buy DVDs and bluray and encode them for iTunes and the ATV. I also use my iPod touch and Ipad to Airplay to my ATV and big screen TV. I have far more content than I know what to do with since I work and have a family as well.