Apache Camel is the most popular open-source integration library. You can implement complex routing, orchestration, transformation, and mediation with this library. With hundreds of connectors/adapters including those to common enterprise backends (file, sftp, jms, web-service, business-rules) as well as cloud services (SalesForce, Gmail, JClouds, Amazon Web Services]) and social services like Twitter, Facebook, and Dropbox, it's not difficult to see why it's so popular. (see the out-of-the-box list at the Apache Camel website component list)
I think one of the biggest reasons why Camel is so popular is that it can run anywhere. It does not require or force you to use any specific container. You're free to choose whatever deployment option you like and are comfortable... including J2EE containers (open source like JBoss Wildfly or commercial like JBoss EAP ), on OSGI containers (JBoss Fuse or Fabric8), or even without a container (yah... containerless.. the new fad.. just a Plain Old JVM).
In this quick video, I demonstrate some cool tooling around Camel provided by Red Hat to help you develop Camel integrations quickly. Though, the emphasis on this video isn't the tooling per-se (I've go more blogs coming up on that), but rather on quickly creating a contract-first web service leveraging the awesome CXF and Camel projects and deploying onto a Java EE container, JBoss Enterprise Application Platform.
The keys here are using Maven Archetypes to quickly create the project, make some minor changes to connect camel and cxf to a servlet that can be deployed into a servlet container, then deploy to EAP. Note, the same strategy will work on Wildfly and Apache Tomcat (or the Red Hat hardened version of Tomcat, EWS).
The source code can be found on github