Diigo - Social Bookmarking

http://www.diigo.com/
Diigo is a social bookmarking site, which is particularly useful as a research and collaborative research tool. The other thing I like about Diigo is the sense of community and knowledge sharing. You can bookmark your favourite sites using the Diigo toolbar, use the web highlighter and sticky notes to bring out the important bits, write a description of the site or tag it, then share it with specific interest groups or your own research list. You can even create friend lists, your own groups and forums on any interests you have.
At the moment Diigo has a thriving community of educators who are sharing resources and ideas on everything from interactive whiteboards, Web 2.0, to project based learning ideas. The developers are also very interested in helping educators to get the most out of Diigo and are currently running forums and consultations to find out how they can improve Diigo for the use of teachers and students alike.