I've had a great experience building web APIs (i.e. returning JSON rather than HTML) in Scala with Spray; it requires you to think a bit more about when things happen (because everything is async), but it's lovely and smooth to use (there's this routing configuration DSL that looks almost like a config file, but then you realize it's all Scala so you can use things like for/yield with it).

In terms of actual websites I understand Play is migrating to run on top of Spray, but I'm not aware of any mature frameworks for doing it right now.