Diigo. Takes social bookmarking / social annotation to a whole new level. It’s been written up in Techcrunch and CNet. No point in repeating the good news.
How helpful is it to bookmark a Web site if you need only one sentence from that 3,000-word article? Diigo is a free bookmarking service that lets you do what we wish Yahoo’s Del.icio.us would: highlight text and comment on Web pages. Diigo caches each site so that you can search within text, not just the topic tags. And you won’t have to leave the Del.icio.us community, since Diigo lets you save bookmarks simultaneously in both places. CNet
One thing about Diigo. One gesture to Diigo can simultaneously update all your other bookmarking sites. That may create a lot of duplication, or it may create the opportunity to connect with others across a world of tagging that remains fragmented. I shall continue experimenting with it.