In no particular order:
- Akka HTTP
- Spray
- Paypal squbs (Akka/Spray)
- DropWizard
- REST.li
- http4s
- Blue Eyes
- Finagle – A fault tolerant, protocol-agnostic RPC system
- Play! and Play-mini! (article) (tutorial)
- Lift / Lift JSON.- makes it simple to provide REST services.