Netty is a high performance event-driven network framework that has support for HTTP and other common protocols. Netty uses non-blocking I/O. But it's pretty low level to use.
Play Framework is a modern web framework for both Java and Scala. It has a very short development cycle - just save your file and update your browser. It's inspired by Ruby on Rails. Play Framework is by default based on Netty (embedded), but it can also be used in a Java Servlet container as a war-file.
I have been using Play Framework in a few months. It's a framework that is easy to use and it has very good performance since it's based on Netty. If you have done some GUI programming e.g. using Swing then you have done event-driven programming before and it's the same principles: Do all long-running processing in a background thread, otherwise your server will be blocked like a GUI is blocked when the thread is used incorrectly.