Akka is a toolkit and runtime that sets out to achieve better performance results with a simpler programming model. It does not compromise between simplicity and performance; its goal is to attain both at the same time. This article, based on Akka in Action shows what Akka is and what it gives you out of the box and then describes the main features of Akka and how these enable you to build powerful concurrent and distributed applications.
What is Akka? (PDF)