Chinese Music has a great little round-up post that lists a few options such as Amigo Fish, iTunes (of couse), and VlogMap. The entry also quotes some people in the space, like CC Chapman, about how they find good podcasts.
The most interesting part of this article by far is that it seems that using directories to find podcasts is falling by the wayside. It seems that listeners are finally becoming aware that podcasting isn't television or radio and the show with the most listeners or most voters isn't always the best show.
While there are a thousand different ways in which a listener may find a podcast, this article is a good primer into some of them.