Here's a screencast of using Akka in the context of Play. Akka's motto is, "Simpler Scalability, Fault-Tolerance, Concurrency & Remoting through Actors". The screencast shows how to implement a hit counter using STM (software transactional memory). It's acting as a library within the context of a Play application. That surprises me (in a good way) since I was afraid you'd have to run Akka out of process. The screencast shows how STM is able to maintain correct results under a heavy, concurrent load. It looks pretty painless. There's another screencast showing how to do the same thing with actors, and then another that shows how to do the same thing for a cluster of servers using remote actors.
Ad maiorem Dei gloriam inque hominum salutem.