Some time ago I did a little paper on folksonomy, and in writing it, I became a true believer in tagging as a means to organize my digital life.
I was first introduced to tagging through del.icio.us and Flickr.
Tagamac is a website dedicated to doing tagging on the Mac platform. I’ve linked there to their page of software. Together really looks nice; I’ve tried Things and love it; I’m thinking of buying Yep and Leap soon, too; both support tagging.
But I’ve also told people that you can tag using the Finder (Spotlight comments) and this tip might help you too. It suggests using a unique character for adding tags.