Crazy Dog Audio Theater
(Including this helpful piece on the basics of writing for audio theater)
Radio Drama Revival
The largest broadcasting company in the world, The BBC, still invests heavily in audio theater. Some of the best modern pieces are coming from here. There are three channels producing this kind of work:
BBC Radio 3 and BBC Radio 4 generally offer drama and more serious fare.
While BBC Radio 7 offers sci-fi and comedy as well as other miscellaneous programs.
The Wireless Theatre Company
Example 1 from WTC (banter, character development and sound effects, including silence): The Importance of Shoes
Example 2 from WTC (language and communication): Laying Ghosts
Example 3 from WTC (framing): The Fun Tom Menace
L.A. Theatre Works (We heard a clip from its version of "Pygmalion")
The Mercury Theatre (Orson Welles's troupe, which created the infamous "War of the Worlds" broadcast
"Art" by Yasmina Reza, three characters talking about a white painting,
From the Rick Emerson side project, "A.Z.," an action piece about when zombies take over the world,
Episode 6: "No Mercy"
And a big list of all sorts of radio theater on the Web