3. Photos (not much has been done here, but that will change).
4. Video (check out the major networks, they're doing a lot here).
5. The new hard to describe (for me) casting done by users on social networks like Facebook (which I almost called Feedbook).
6. I think Twitter is a form of casting (it's also a catcher).
7. Publishing bits of code in feeds. I use this extensively as the update mechanism for all my software. Others do too.
8. Torrent feeds (slapping forehead). With this innovation it's possible to write a TiVO that runs on a desktop or Mac Mini. Check out EZTV's feed, very sensible, futuristic (one hopes).
9. What else?
For MP3s you're basically on your own. There aren't many tools for creating RSS 2.0 files with MP3 enclosures. This art has been around since 2001, it's been popular since 2004, so it's fair to assume perhaps that there isn't much demand. It's pretty easy to cobble together a podcast feed by hand. I write scripts to do it, myself, I never do them by hand.
Blogging software is probably the most common tool for news item casting.
For photos, you have Flickr, and Apple's iPhoto does something they call "photocasting" but I haven't investigated this yet (I will, for sure). I have some stuff coming here myself not too far down the road.
Question: What photo collecting sites offer RSS feeds of users' photos? Are they compatible with Yahoo's feed format (they use a namespace called Media RSS) or do they use enclosures, or something else?
Answers: Zoomr, ShoZu (but no metadata about the photos, so what's the point).
For video, it's basically like MP3, if there are any tools on the casting side, I'm not aware.
If you'd like to add anything to this list, which is far from complete (I'm sure) please use the comments.