ViCasting is a broadly general podcast directory.
They seem to use a pure tag oriented categorization. The front page has a googlesque minimalistic presentation of just a search box. But there is a Browse link which takes you into tags. They have so many tags they're broken up by the letters of the alphabet.
For example, I click on Aluminum and find there is a Vintage Airstream Podcast covering all things related to the Airstream travel trailers. I wouldn't have thought that someone could make 30 episodes of a podcast about Airstream Trailers, but someone has done so.
The show listings include a show description, and descriptions of the listed episodes.
The site seems pretty well organized and useful. Not very flashy at all.