For Listeners:
Podcasting directories are the best way to find podcasts. A good directory will be categorized either alphabetically or by topics. Not all directories are the same. Some directories focus solely on certain themes. For instance, MusicOnlyPodcasts.com, is a directory only for music, while Podcastdirectory.com, is a general directory that covers all themes.
Probably the largest and oldest directory on the web is Podcast.net. They have a large database of podcasters, and the largest theme index of them all.
For Podcasters:
The way to get heard and receive traffic, is to find relavent directories and ask to have your site or podcast listed in their directory. Most are pretty automated and allow you to fill in your own show information. Some will approve you immediately, some will will make you wait while they review your site. A NASCAR directory is not going to allow golfing podcasts, and vice versa. Feed Shark helps podcasters submit show information to multiple directories and internet locations.