Podzinger is impressive for its features. Search results include text highlights of the portions of the audio that match your search. Click on any word in the highlighted text, and the podcast begins to play at the point where you clicked. To the left of the results is a control panel that lets you play the entire podcast or move backward or forward through the audio. The control panel also allows you to download the entire podcast, subscribe to its RSS feed, or subscribe via iTunes or Yahoo!.
Podzinger uses speech-recognition technology to create a text index of the audio data in any MP3 or WAV file. An FAQ says that the searchable text of a podcast generally becomes available within eight hours of the podcast’s publication.
Its weakness, at least so far, is in its scope. I searched for several law-related podcasts I listen to — including my own, Coast to Coast — and did not find them.
One other full-text podcast search tool I know of is Blinkx. Various sample searches suggest it is broadest in its coverage of the three podcast search tools mentioned here.