I downloaded a WordPress theme recently that introduced me to Ultimate Tag Warrior, a tag management plugin for WordPress < 2.3.

Ultimate Tag Warrior is a multi-headed hydra of a plugin. It allows you to add tags either through the Write Post page in wordpress in a tag box, on posts using an AJAXy box, and in posts using special syntax from external editors (or internally, if you’d like). From the write post page, you can also get suggestions for tags using the Yahoo! keyword suggestion service. Once you’ve got your tags entered; you enter a realm of interesting ways of doing things with your tags.