If you want to provide or consume HTTP-based APIs with Scala (or Java) then this talk is for you: Akka HTTP takes reactive high-performance web interactions to the next level. Fully asynchronous and non-blocking, a type-safe and very powerful declarative API, an immutable HTTP model without compromises, all these are perfectly fitted to the Actor world.
Among many smaller improvements over its predecessor library spray.io, akka-http comes with one major advantage: it is based on a new abstraction called "Reactive Streams” which provides a number of important benefits, not only for working with HTTP.
Overview of spray, "a suite of lightweight Scala libraries providing client- and server-side REST/HTTP support on top of Akka."
spray has been aquired by Typesafe and will soon be integrated into Akka as akka-http.