I've been hitting the random link a lot lately, tagging old weblog/photolog entries with multiple categories.
This photo in particular makes me wonder how I ever made do with only one category per entry: I managed to think up no less than 12(!) fitting categories for it.
[1]http://www.adaptivepath.com/publications/essays/archives/000361.php
Although I have a single textbox per entry wherein I type the tags, on saving they are extracted and stored as separate category references in the db. This allows me to easily select+autofill from a quicklist of previously entered tags, thus preventing duplicates/spelling variants.