This is a really useful article if you're interested in learning How to Analyze your Site with Del.icio.us. I've mentioned del.icio.us a few times as a search tool, and Ann Smarty has provided some great examples of what can be done. For example, it's possible to use Delicious to search a specific website page using http://del.icio.us/url/ Doing this for my home page shows that it's been saved by 153 people, with the tags Internet, library, resources and search being the most popular ones. On the other hand, my Web 2.0 I want to page has been saved over 6,000 times, with tools and Web2.0 being the most popular tags.
Ann also suggests using Similicio.us to find related sites. Google's done this for a long time of course, but it's always a good idea to have an alternative. Finally, a Firefox plug in, SeoQuake, which provides more information to web authors gives more content as well.