This is the one I use Michael:
WordPress › Simple Tags WordPress Plugins
Any cool plugins to generate the tags for all posts in a blog? I had one that took from the yahoo and it worked fine, but after the recent wp upgrade it stopped working.
Looks good, checking it now
Hm how do you get it to add tags to the existing posts? Any way to add them to all posts automatically? I got over 100 posts here
I don't remember how I've done it, but I think I've converted categories to tags or whatever, in any case I think that the plugin has some kind of engine to find matches for your tags inside each post body.
(Can't check how atm)