spray was influenced by many projects, but the following ones especially stand out:

Another functional web service building toolkit for Scala. sprays HTTP model was originally based on the one from BlueEyes and the core ideas of the routing DSL are also heavily inspired by what the BlueEyes team made available.
Netty is an excellent choice if you need to build asynchronous event-driven network applications with Java. spray adopted Nettys upstream/downstream pipelining approach for its spray-io module.