When I was a kid, I often stayed at my grandma's house during the summer. While I was there once, she introduced me to old-time radio and I loved it. When I got an ipod, I wondered if anyone was making new old-time radio style shows that you could download. My search led me to Pendant Audio, who has a bunch of original and fanfic-style shows.
I highly recommend their DC comics based podcasts, which all run together. They have Superman, Supergirl, Batman, Catwoman, Wonder Woman, Martian Manhunter (a 6-episode miniseries, with the last two being part of a crossover), and now Green Arrow.
I haven't checked out their original stuff, but I think Seminar sounds the most interesting. They even accept scripts from fans!
Here's the link to their info site. All their stuff is on iTunes: click here