Java API for XML – Restful web services (JAX–RS) is the standard specification for developing REST web services.
JAX-RS uses annotations to simplify the development and deployment of web service clients and endpoints. Important JAX-RS annotations can be used @ http://javajee.com/important-jax-rs-annotations.
Apache CXF, an open source Web service framework
Jersey, the reference implementation from Sun (now Oracle)
RESTeasy, JBoss's implementation
Apache Wink, Apache Software Foundation Incubator project, the server module implements JAX-RS
Since all these implementations are based on the JAX-RS API, you can easily switch to another implementation anytime.
Jersey is a popular JAX-RS implementation. Jersey is the open source, production quality, JAX-RS (JSR 311) Reference Implementation for building RESTful Web services.
You can download JAX-RS from Jersy Homepage at jersey.java.net.
In addition to being the Reference Implementation, Jersey also provides an API so that developers may extend Jersey to suit their needs.