Twitter Commons uses Jetty, and provides a lot of the glue and miscellaneous parts of a web service, like logging, statistics, registration, and lifecycle management. On top of Jetty we use Jersey and Jackson, which is a tried and true combination that is also used by other stacks like Yammer’s Dropwizard. Jetty is an incredibly fast embeddable web server and servlet container. Jersey is the reference implementation of JAX-RS, the Java specification for writing REST web services using simple Java objects and annotations. Jackson is the de facto standard for fast JSON processing on the JVM. In addition it also uses Google Guice, the very well-designed dependency injection framework. It takes care of all the wiring in your application and saves you from writing FactoryFactories and other fun things. Dependency injection makes your code modular and easily testable, and Guice makes dependency injection easy to write.
(Full Story: http://nerds.airbnb.com/httpjson-services-in-modern-java )