I was impressed by the preview I got in September of the bookmarking and Web annotation tool Diigo 3.0. It's taken the company until this morning to release this version to the public. In the interim the team has added features and tweaked the design. It's been worth the wait. This is a powerful and deep tool for serious Web users.
As before, the service has a plug-in (I tried it in Flock) that lets you clip and save text from Web pages, or just page URLs themselves. You can categorize and tag your findings for later, and keep your stuff private, shared with friends, or make it public. It's also very easy to annotate pages and send your mark ups to other users. Diigo is not the only product that does these things (see ClipMarks; BlogRovr), but this version's implementation is especially strong, which is not something I said about previous releases.
There's also a very nice new browser sidebar that shows you a lot of useful and focused information, including your own latest bookmarks, those from your Diigo friends, and most cleverly, the Diigo users who have also saved information from the page you're visiting as well as the site itself.
Diigo takes all the data it collects from users and lets them rotate it in interesting ways. When you're looking at the page for a site, for example, you can easily see what other users who bookmarked that site also bookmarked (sort of like MyBlogLog, Medium, eSnips). The system will suggest other sites, based on the tag cloud for a site as well as the affinity you have with its other bookmarkers, that it thinks you might like (see also: Twine). There's also a social angle: The system suggests people that it thinks you'll find interesting, by identifying the ones closest to you in bookmarking behavior.
I recommend Diigo, especially with the plug-in. It's a complex tool but if you take a few minutes to learn what it can do and how it does it, it can make you a more productive, smarter Web user.