Resteasy is a JBoss.org project aimed at providing productivity frameworks for developing client and server RESTful applications and services in Java.  It is mainly a JAX-RS implementation but you'll find some other experimental code in the repository.

JAX-RS
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The JAX-RS code is in the jaxrs directory.  After pulling down a clone of the Resteasy repository, cd to jaxrs/ and just do mvn install to build.