[lmm]

Scala/Spray. Wonderful full-fledged type system, complete with a strongly typed model of HTTP (so that e.g. ContentType is well-typed, and takes a well-typed model of a MIME type, not just a String). Excellent concurrency support, very fun, all the JVM tooling/debug support. Deployment needs a tiny bit of fiddling - I recommend building with maven and using the maven shade plugin, then you get a standalone jar that you just need a JVM to run.

The unique selling point is that your routing is defined in a DSL that's almost as nice as the config file most systems would use - but it's ordinary Scala, well-typed and refactorable in the normal way because everything's just code. And because the type system has higher-kinded types you can use typed wrappers to represent cross-cutting concerns, with the lovely for/yield sugar for composing them. E.g. you can have database-session-in-view but in a principled way: you have a wrapper type that represents an operation that needs to happen in a database transaction, and if you want to compose two or more such operations you use the for/yield sugar, and it's all refactor-safe and you can tell which unit tests need a database (fake or real) because it's right there in the type. And at the route level you either have a marshaller that handles your wrapper completely transparently, or your own directive that works exactly like the built in ones because again, it's all just code. You can click through to the source of the directives and see how they're implemented, not just magic annotations like with JAX-RS. If you want to get super mind blowing you can even use for/yield to compose together directives to make a custom directive.