There are a few good frameworks in the integration space, but Apache Camel really stands out.
Camel is explicitly focused on simplifying integration tasks and making users more productive. It is heavily based on the EIP patterns described in the famous book by Hohpe and Woolf and provides a DSL based on these EIPs. The DSL helps expressing and communicating the intent of the integration tasks.
Camel is written in Java but supports a number of other languages like Groovy, Scala, Python, Ruby, JS. It also supports a large number of protocols, from accessing file systems and raw tcp/ip to http, ftp, smtp, xmpp, irc to soap and web services via Apache CXF. With over 60 components as of now Camel is the most comprehensive integration framework in the open source space and is available under the liberal Apache 2.0 license.
Camel is a mature, robust project already used in production by large companies. Its strong community highly appreciates new ideas and contributions.