Simple Tagging is another tagging plugin for WordPress: smarter, better, faster :-) Tag Cloud, Related Posts, Related Tags, Tags Tabs (new!) and more.
For WordPress 2.3 and upper : Please try and use Simple Tags Plugins
The two mostly used tagging plugins are the Ultimate Tag Worrior (UTW) and Jerome’s Keywords.
I think most people use UTW, however especially its performance has been criticized in the past. Also, it does not provide a auto-complete type-ahead dropdown menu when tagging posts but it always displays all tags of the entire WordPress installation which can be quite annoying if you have really many tags.
Jerome’s Keywords is unfortunately still in beta and it seems that the project will not be continued.
With Simple Tagging Plugin, I’ve coded another tagging plugin for WordPress which is based on Jerome’s Keywords plugin and can '''import''' all your tags from UTW or Jerome.
It provides you with everything you need for tagging, in the following a list of the features: