Freetag: PHP/MySQL Tagsonomy pluggin
At last someone has developed an Open Source tagging/folksonomy system for those that want to play - although the concept of tagging is not hard, and for an experienced PHP/MySQL dev certainly not rocket science to code, isn't it nice that someone's built a tagging API for you?
Freetag is an easy tagging and folksonomy-enabled plugin for use with MySQL-PHP applications. It allows you to create tags on existing database schemas, and access and manage your tags through a robust API.
There's a module in the drupal CVS for folksonomies but the guy who wrote it is an incompetent, and i wouldn't trust him to code "hello world" - i might see if i can work it into a test version as a module, could be fun :-)