I am sure there are things like this out there, but when I was doing my simple stats for flickr tagging, I took the very inefficient method of paging through the flickr tag pages and listing unique user names.
Surely there is some flickr tool that can tell how many unique users have used a specific tag? I picked and poked but could not find it. There are plenty of things to return data per individual account, and lots of cool way to render public tags.
Same thing for del.icio.us- surely there is a web tool to show how many people have used the same tag??