This site creates a tag cloud given any url:

http://www.yvoschaap.com/tagcloud/index.php?filename=blog.outer-court.com

+ you can get the source code to make your own.

They don't use their own crawlers, they cluster search engine results, that is all. Another player in this field is Carrot search (look for Carrot2).

Clusty does have its own (Vivisimo) crawler. That is how the Shakespeare plays and sonnets were indexed. The meta-search at Clusty.com calls ten or so search engines, including a few set up by Clusty by crawling Wikipedia, Open Directory, and others.