Web APIs galore

With many popular websites offering their services to be extended programmatically, this is a good time for Web developers and hobbyists.

Here is a list of such APIs:
Technorati
Google
eBay
Yahoo
FedEx
Flickr
BBC
EVDB
Backpack
Amazon

The MSN Messenger Activity API is one of the many MSN API's that gives you innovative tools to build applications that integrate with MSN Messenger. There is now also a contest to build the best app on MSN Messenger.

Update: The website ProgrammableWeb has a great deal of information on mashups and the new Web 2.0 APIs.