We started out by using PyBlosxom's categories here on Copia, but they were clearly never going to quite cut it. We needed multiple categories. Thankfully I found Bill Mill's keywords plugin (this is probably a more permanent link). It's one of the few PyBlosxom modules I haven't had to hack to get working. I did have to spelunk in his code to figure out that the $keywords story template variable has all the keywords for a specific story: this part was not documented in the doc string of the plug-in (it mentions the $all_keywords variable, which is more useful in other templates in a flavor).