There are tools that analyze visitors’ behavior and generate the list of resources people also like to visit (Alexa.co), there are tools that analyze users’ bookmarking preferences and generate the list of sites having similar tags when bookmarked (Delicious and FoxMarks).
"Similar site" is the tool that is also "mainly based on user annotations or tags on websites". While that remained not really clear to me which database the tool is using (probably it’s own) but the results are intriguing enough to share and the interface may look a bit simplistic but the actual job seems pretty well done.
So: type in any page URL and hit "Search". The results page returned contains the following (useful) information:
* The confidence level (more about this one below)
* The most frequent tags used for the page (you can exclude any or add your own to refine the results);
* The list of similar sites;
* Most frequent tags characterizing each page in the list;
* The similarity score (how similar each page appears to be to the base one, in %);
* The quick button to find similar pages to each one in the list.