podcast.com is probably the best name anybody could have to use for a podcast directory. Lucky guys.
There is a notice on the front page saying they're working on revamping the site. Of course this review is based on what I see today.
They use a multilevel hierarchy of topic categories. Browsing the topic hierarchy is an AJAX/DHTML style experience, where you click on topic names and the index automatically expands in place to show subcategories.
Anybody can add a podcast to the directory using the form on the front page.