The "Top 25" sites foster the qualities of innovation, creativity, active participation, and collaboration. All of the websites are free, Web-based sites that are user friendly and encourage a community of learners to explore and discover.
Top 25 Websites for Teaching and Learning
I've used several of these.... I can speak for the power of VoiceThread, Wordle and Wikispaces.
Another great site not to make the list is Wallwisher.