Diigo is a new social annotation/bookmarking tool. In one respect it is similar to del.icio.us. It even takes advantage of the del.icio.us API so that items that you tag or bookmark with Diigo, are also tagged to your del.icio.us account. Where it is quite different from del.icio.us, is that you can also more fully annotate your bookmarked pages, and bookmarks can also be saved locally. You can highlight and tag specific images and paragraphs from web pages, and also add sticky notes. These sticky notes can be public or private. For example if I tag and add a sticky note to a page, I can also see other sticky notes left by others or I can send a notification to a colleague so that she can read my annotation and respond. I can also use Diigo to save my selections and annotations and review them later. I can also forward them on via email. The Diigo toolbar puts all of these tools at your disposal.
The Flash Tutorial gives a very good overview. The more I play with Diigo the more it looks like an interesting alternative/compliment to del.icio.us.