Web 2.0: Cool Tools for Schools is a site I learned about from NCS-Tech (which is a great edtech resource too). Cool Tools for Schools has a listing of hundreds of educational technology and web 2.0 tools that teachers and students can use in their classroom.
The resources are sorted by categories such as "presentation", "music", "collaborative", and "research". There are 18 categories, plus a "Teacher's Resources" page with lesson plans, books, games, and more.
I spent about an hour going through the site and found it to be a great place to go for resources. Definitely a site to bookmark.
Another good resource for finding Web 2.0 tools is Go2Web20. Cool Tools for Schools is organized for educational uses, while Go2Web20 is set up for any industry.