QUIET, PLEASE is exactly one of those types of sites. The main page gives an overview of the shows history and it's creators Wyllis Cooper and Ernest Chappell, who "starred" or narrated the episodes. Chappell BTW sounds remarkably a lot like KGO AM radio personality and cranky blowhard Dr Bill Wattenburg.
What really makes the Quiet, Please website shine is the fact that many episodes, over 100 I believe, are offered online along with one to several paragraphs of info or synopsis about each radioplay.
Another really great resource for tracking radio dramas, especially in the sci-fi, horror, fantasy realm is the OTR Radio Plotspot. This is almost like a graphical wiki of 25-30 different series, with plot synopsis and reviews, some of which are submitted by readers. I find this to be a great resource when wading through the massive pile of OTR mp3s and wanting to find a certain or particular type of show. The plotspot also has a great index of known authors whose work has been adapted to radio drama.