The people behind the 'Programmableweb' made a great lists on Web 2.0 API's and sites that combine (Mashup) Web 2.0 Api's! Check it out:
http://www.programmableweb.com/mashups (in a alphabetical list)
http://www.programmableweb.com/matrix (in a great 'matrix' view)
When all these services will get together, things are beginning to look really interesting..
I would also like to link to this post where you can find a good collection of links related to the Semantic Web (and RDF).