Podcast Inspector allows you to search multiple podcasting sites at once.
Below are some of the sites that Podcast Inspector searches:

PodcastPickle.com	libsyn.com	SchoolOfPodcasting.com	PodFeed.net
PodZinger.com	PodcastDirectory.com	PopCurrent.com	Odeo.com
AmigoFish.com	WomenInPodcasting.com	Pod-Planet.com	VitalPodcasts.com
Podiobooks.com	podcast411.com	Podshow.com	Idiotvox.com
Feedburner.com	PodioBooks.com	ThePodLounge.com.au	PodcastAlley.com