[tag]UTW[/tag] is the most popular plugin for tags. I just upgraded to version 3.14159265 when I saw this post. I regretted it.
I have seen comments about UTW having trouble with WP 2.1, that didn't bother me as all my themes are OK. With 3.14159265 tags didn't show up. Categories didn't convert to tags.
I have just gone back to a working version which is 3.1415926 downloaded from here. Lucky that I could always go back.