Edudemic, a great site for educators, has a listing of The 100 Best Web 2.0 Classroom Tools Chosen By You.
The site polled the Twitter followers, Facebook fans and asked for submissions on the site for the best Web 2.0 classroom tools. They had over 900 submissions and 100 different Web 2.0 tools, so they posted those 100, in no particular order, with a brief description of the tool.
This is a great place for educators to go and find some great resources to use in their classrooms.