Spray
Scala libraries for building and consuming RESTful web services
...The core philosophy behind Spray is that it should act as a “library” for integration and HTTP layers, not as a full application framework—it gives you the building blocks to handle HTTP and REST but doesn’t impose heavy structure. It includes modules for low-level HTTP I/O, routing, client and server APIs, HTTP model (requests/responses), (un)marshalling, servlet adapters, and testing utilities. Its routing DSL offers expressive combinators for defining HTTP endpoints, and its testkit allows route logic to be tested in isolation (even without spinning up actors). It was popular in earlier Scala/Akka stacks but has since been superseded by Akka HTTP as the maintained successor.