iCyte is a wonderful site for bookmarking and annotating web sites. Also, a user can save a site that is no longer active on the web, comment on them, and then send it to others or embed in their site.
I highly recommend giving iCyte a try by clicking here!!
For an ambitious project on 21st Century learning on iCyte, I suggest checking out Duma Lucian's, Free Project in the Clouds for Teachers around the World.