Diigo is a social bokmarking tool.
If you have been using dili.cio.us or similar bookmarking tools, you will find it to be useful as it allows you to highlight text and add comments on that for future use on any page over web. It inherits almost all the deli.cio.us features plus few more. If you bookmark a page, it caches it.
I am still going through its features and wud like to give a chance to others also as it is in beta stage and u can join only if you get an invite. So comments are open and I will send invites on demand.