The Sonic Society is a clearinghouse for small independent radio theater troupes all around the world. Some of my own favorites for passing the drive time: Decoder Ring Theater and Icebox Radio Theater.

There are many many more, ranging in theme from attempts at recreating the old-time radio feel (Decoder Ring Theater) to experimental sci-fi (19 Nocturne Bvd). Most of them provide RSS feeds for your favorite podcatcher. Did I mention that it's all free (or perhaps "NPR-style beg-ware")?