A Podcast Like That
A Podcast Like That doesn't have a topic directory. Instead you search and it shows you matching podcasts.
Each podcast has its own page, which lists overview, and lists the items in that podcast. The items are presented pretty well, with text taken from the feed.
One nice tweak is they have a "related podcasts" feed which is autogenerated somehow from topics.
There are "tags" attached to each item and each feed which then goes to a page listing items related to that tag. The tags and such form a way to browse around the site in topically relevant ways.
Anybody can enter their feed in the "Ping Us" page.