The Restlet Framework has been available since 2005 when it was the first RESTful web framework for Java. It has support for the JAX-RS API, but its own Restlet API is both client and server-side since day one, much more comprehensive and extensible. We are free to innovate based on our community feed-back, without having to go through lengthy JCP standardization processes.
Also, we just published the 'Restlet in Action' book last September along with its version 2.1. Our internal connector is fully asynchronous and based on NIO and we are constantly stabilizing it even though it isn't ready for heavy productions yet (use the Jetty connector or a Java EE container with no change to your Restlet application).
Its consistent support for Java SE/EE, OSGi, Android, GAE and GWT with dedicated editions is unique. A port to JS (Node.js + AJAX) is also under way. We have also started work on version 2.2 with the first milestone being released (with full Java 6 support, OAuth 2.0 extension based on the final spec, etc.).
In term of references, we have many large companies using it including LinkedIn (see their GLU open source project), IBM, NVidia, ForgeRock, NASA, Sonatype, Apache Camel, Mule ESB, etc. Google has been using it internally as well. See some quotes here:
http://restlet.com/discover/quotes
In January, we'll launch a new community web site as well as APISpark, an all-in-one platform to create, host, manage and use web APIs, directly based on Restlet Framework (PaaS), so the project is active and has an exciting future!
Best regards,
Jerome Louvel
PS: I'm the Restlet Framework creator and lead developer.