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here's Diigo, with communities sharing bookmarks, clippings, annotations, ...

The social annotation service introduced by Diigo allows users to add highlights and sticky notes, in situ, on any web page they read. Imagine a giant transparency overlaying on top of all the web pages. Users can write on the transparency as they wish, as private notes or public comments. And they can read public comments on the transparency left by other readers of the same page, and hear their "two cents" and interact with them.

Personally I found Diigo www.diigo.com to be the most delightful and one step ahead. This fun tool allows you to highlight, bookmark and forward, any content from any site on planet Web. Even better, you can actually add a sticky note with your comments for either public or private viewing. This is in addition to all the regular features of a wiki – creating groups, shared content, collaborating on files and projects etc. You do need to download the tool bar which comes in all popular browser versions. I spent a most happy half hour, sticking notes on random web sites, but am sure it can be put to more productive use. Public comments are monitored by an editing team.