We've talked before about the many benefits of social bookmarking (Digg, Del.icio.us, StumbleUpon, and so on) for sharing the web pages you've discovered and for helping others to share your own site or blog. But when it comes to online research and collaboration, are the usual social bookmarking services really the best possible tool for organizing that flood of online information?

If you've bookmarked the same website several times over, or misplaced an important bookmark among a hundred others in your list that share the same tag, the limitations of tagging as an organization tool will be clear. And if you're like me, sometime you'll find a certain bookmark easily enough, only to discover that you can't remember precisely which single sentence it was that made you bookmark the page in the first place!

Diigo.com takes social bookmarking to a new level of usefulness.  
 
It's all about social annotation. Highlight text or leave a note, right on the web page itself.

Make your notes and comments public, share with selected groups, or keep them private for your own reference.  

Trying to find a bookmark is like seeking the proverbial needle in a haystack? Diigo goes beyond tags and folders, letting you search within the actual text of cached pages to find the one you need.

Install the toolbar on your Firefox or Internet Explorer, if you'd like to — but there's also the lightweight option of a simple bookmarklet. The drag-and-drop "Diigolet" button gives access to all the main Diigo functions with just one click from your browser's existing Links or Bookmarks toolbar.  

Bookmark a page, add tags, highlight text, add sticky notes to the page, add a summary note about the page in general — public, private, or shared with groups. When you return to any webpage that you've previously bookmarked and annotated, a click on the Diggolet button will show any notes and highlighting you've added, as well as any public or shared-with-you notations made by other Diigo users.

For research and writing collaboration, in particular, I'm finding all of this a real time-saver.  Normally, for example, you might point your colleague to a social-bookmarked page you'd like them to read — but then you'll often need to take the time to explain its significance in a separate phone call, chat message or email.

With Diigo, instead, just bookmark a page, tag it with the name of the project you're working on, and share it with a custom-defined group of collaborators. Highlight the passages to which you'd like to draw attention, and leave a private note with further instructions: "Please fact-check these points and get back to me," for example, or "Could we use this data in our year-end report?"

And if you're already deeply committed to Del.icio.us for your social bookmarks, no problem — Diigo lets you save a bookmark in both places simultaneously, so there's no need to maintain two separate lists of bookmarks or to choose between the two sites. You can also import your existing Del.icio.us bookmarks and browser-stored Favorites, to get the benefits of social annotation for all your bookmarks.