Web 2.0: Cool Tools for Schools is a very extensive list of Web 2 tools and applications. One thing I like about it is that it's alphabetical, so you can quickly find something if you know the name of it. There are hundreds of links here, to all kinds of tools. There's also a category list, which is helpful if you don't know the name of a site or if you know what kind of tool you need.
This wiki could be a black hole for time, though. There are so many tools on this wiki, that you could spend days clicking on links and exploring them!