I listen to all sorts of things, mostly music. I found that while some podcasts can be great to run to -- very distracting! -- because the podcasts I listen to vary lots from week to week, with different lineups and guests and topics, it's hard to know in advance that you're going to like an episode enough to run with it...
Music is generally good -- I got through most of the Couch to 5k listening to various bits of music; I listened to lots of the more bouncy and energetic end of the kind of rock and pop I normally listen to. I found the iTunes Genius quite good for this -- I would find one good song for running, and hit the Genius button, and it'd generally keep the right feel going.
On the other hand, some things it came up weren't that smart: I have a couple of Madness albums in my library. And if I thought "Driving in my Car" was inappropriate for running, that was only until "Cardiac Arrest" started playing...
I also used a Mac app called
Tangerine. It analyses your music library for beats per minute and strength of beat, and you can set it to generate random playlists of music with a good beat, that start off slow, for example, and gradually build up. Had some interesting results from that, many of them good. It's not that brilliant to use, and it won't work on DRM'd music (the older, non-iTunes Plus stuff), but I think there's a free trial...
I've also found that the kind of music I listen to when I'm running is different from the kind of music I listen to when I'm not. I don't normally like the more dance-y/electronic end of pop music, but I find Lady Gaga pretty good to run to
Matt