If you go here...
http://www.audible.com/sp/2for1
...you'll get one version of the 2 for 1 credit sale that expires today with three categories, Chilling Thrillers
Summer School
Summer Lovin'
Then there's the "secret" sale (with different books than the sale above). I got an email for a Romance sale (with a very small selection). This sale ends 7/31 and supposedly only works for folks who got the email.
Here's the link in case it works for others...
http://www.audible.com/sp/2for1_Rom
EDIT: With a little URL manipulation I figured out the one for the SciFi sale...
http://www.audible.com/sp/2for1_SFF
and the Mystery & Thriller sale...
http://www.audible.com/sp/2for1_MT
All the secret sales end 7/31 and don't seem very "secret". Even though the page says "Remember, the only way to access this sale page is via the email banner link that brought you here" that's obviously not true.