I am a big fan of Google, and Microsoft Live. These are the two services that I use the most, but if I need to search through the contents of a podcast then I will use a service called PodZinger, this is the best search engine that I found for it.
PodZinger uses Speech-to-Text technology, to make audio and video searchable. Most search engines rely on the metadata provided by the shows creator, but it doesn't allow you search the audio of the podcast itself.
Another very cool feature of the site, if you want to listen to the search results all you have to do is click the link and it will take you to that point in the podcast. You can also listen to the whole podcast if you want.