A family of languages (not a specific language) based on the Actors Model.
Some of these languages are described by Gul Agha in Actors: A Model of Concurrent Computation in Distributed Systems (The Mit Press Series in Artificial Intelligence) ISBN:0-262-01092-5.
Actors are objects that change role and send messages to mailboxes instead of directly to each other. They are a very elegant distributed programming concept.
Some discussion of the actors model that was here previously has been moved to Actors Model.
Steve Dekorte maintains a list of actor languages at wayback.archive.org 
Plasma Language and Act Language (Act 1, 2 and 3) are the original actor languages designed at MIT by Carl Hewitt et al.
Salsa Language is an actor dialect of Java Language.
Ee Language is an actor language that focusses on the security benefits of the actors model.
Gerald Sussman and Guy Steele's attempt to understand Carl Hewitt's Actors Model resulted in the creation of the Scheme Language. A language based on Scheme that is even closer to the pure actors model is Double You Seven.
Sussman and Steele arguably got continuations wrong; continuations in the actor model do not have the security problems that caused them to be omitted from Double You Seven. In particular, Scheme continuations can return more than once to code that is unprepared for this. In the actor model, OTOH, an actor can be written to respond a continuation more than once, but that would have to be explicitly programmed. Also note that actor continuations are a derived concept, whereas in implementations of Scheme they have to be supported as a primitive.
Cal Language (also called Caltrop) (not really... but it uses the word 'actor' to describe Dataflow Programming units.)
Io Language (but not Original Io Language, which despite being based on continuations is sequential).
Erlang Language (Erlang processes correspond approximately to actors, although in the current implementations port/process IDs can be forged).
The Actor Foundry is a Java implementation of actors.
Toon Talk is an actors-based Graphical Programming Language.
Jo Caml is based on the Join Calculus, which is equivalent to actors.
Cee Omega is also based on the Join Calculus.
Pict Language is based on a variant of the Pi Calculus. The differences between this variant and the usual Pi Calculus make it much closer to actors.
Ac Talk is a Smalltalk library (available for Squeak) the implements an Actors workbenck.
Not the same as the circa 1990 Whitewater Group Win16 pascal-like OO programming language Actor Language.
Also see Communicating Sequential Processes, Join Calculus, Pi Calculus, Object Capability Language.
See original on c2.com