Erlang. Erlang is a functional programming language for developing distributed fault-tolerant applications with soft-realtime requirements. Erlang comes with a powerful library supporting common architectural patterns.
The distinguishing feature of Erlang is the combination of the functional paradigm with the actor communication model. Every recursive function can be easily turned into a process that communicates via asynchronous messages with other processes. The only state in an Erlang program is the state of the communication mailboxes. Hence, no variables are shared. Therefore, functions or processes can be safely replaced during runtime.
Furthermore, Erlang comes with its own virtual machine that fully exploits multi-core processors. Erlang demonstrates the advantages of a declarative language when developing distributed applications.
As a starter I recommend the book of Erlang's inventor Joe Armstrong:
For a deeper understanding of Erlang's framework library OTP I recommend:
Have fun with Erlang!
P.S: Erlang is not just fun. Amazon's distributed database web-service SimpleDB is written in Erlang.