Here are some of the most popular web sites that are doing tagging right now.
* del.icio.us - a social bookmarking system
* Flickr - a photo publishing / sharing site
* Technorati Tags - a feature added to this popular blog search engine
* MetaFilter Tags - a feature added to the original group blog.
* BlinkList - personal and social bookmarking
* 43 Things - a social networking site that is built on tags
This Rollyo.com (as in Roll Your Own Search Engine) search bar that TJN rolled includes one called "Tag Soup" - it checks all of the above sites for a particular tag or term(s).
Tags & Folksonomies - What are they, and why should you care? [threadwatch.org, Jan. 11, 2005]
Folksonomies - Cooperative Classification and Communication Through Shared Metadata