I recently did a linkdomain:seobook.com search on Yahoo! and noticed the search results had "saved by X people" near many of the top listed backlinks.

Results that are bookmarked are typically sites that are frequently visited by real people. As the web becomes more read write the "saved by x people" on the top backlinks will be a great quick SEO tool for testing the depth of support for your site or a competing website.

I consider this website to be somewhat legit, but my sites I would classify as less than legit have no backlinks from pages that are bookmarked on other sites, which is another way of corroborating the value of Jim's philosophy on finding the best pages to get links from within a site.

If you just search for a site it I believe Yahoo! returns those results roughly in order of authority and shows which pages were tagged in those results.

Although more information existing online increases the bar for what is required to be competitive / remarkable / linkworthy I also believe that search engines sharing as much data as they do still makes marketing easy. Thanks to them for that.

The two big downsides to tagging are that it is easy to spam, and currently few people outside of exceptionally techie circles use it much.