If you haven't gotten into OTR (Old Time Radio) yet,
www.archive.org has tons of old Radio Dramas that are produced alot better dramatically speaking (at least most of them) a few that I recommend are
Sci Fi:
http://www.archive.org/details/OTRR_Dimension_X_Singleshttp://www.archive.org/details/OTRR_X_Minus_One_SinglesFor Horror/Suspense some really good ones are
http://www.archive.org/details/OTRR_Suspense_Singleshttp://www.archive.org/details/Escape_HQ_192That being said, the best NEWER Audio Drama I've heard, aside from the World War Z audio book dramatization is
http://www.archive.org/details/OTRR_Claybourne_Singles For other random Audio Drama,
http://www.archive.org/details/VoyageOfTheScarletQueen - a really well done Nautical themed drama with pirates, cannibals and gunplay based in the early to mid 1900s
For Comedy, one of the best duos from the Vaudville age that made it into and successfully on Radio
http://www.archive.org/details/FibberMcgeeMolly1930sOr the ever present Mel Blanc, master of Voices and in a ton of Bugs Bunny/WB cartoons:
http://www.archive.org/details/OTRR_Mel_Blanc_Singles