I've been listening to a bunch of podcasts lately and have become a huge fan of Radiolab, from WNYC. A lot like This American Life, where each show has a theme and an eclectic mix of subtopics.
Here's their homepage: http://www.wnyc.org/shows/radiolab/
So far, my favorite shows are:
The Bus Stop (March 23)
Limits (April 5)
In C (December 14)
And there are a bunch of really neat ones on animal minds as well.