I've been collecting webapp APIs. Here's what I have so far. I'm bound to have missed some. Drop me a line or leave a comment if you know of any others.

• Yahoo!
• Flickr
• 43 Things
• Bloglines
• Upcoming.org
• eBay
• Webjay
• Typepad Atom API
• Audioscrobbler
• A9
• Amazon
• Feedmap
• Simpy
• hostip
• Backpack
• del.icio.us
• Google search
• Google adwords

Oh, and if you think the links look odd, I'm also experimenting with my own tagging webapp.