This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma on-line! See this thread for details.
What we see however is that the two most popular at this moment are Mule and ServiceMix.
And new interesting one though which has recently gained a lot of attention is Spring Integration.
Probably the easiest ESB to get started with is Mule. It provides a low entry level, very powerful ESB. If you're interested in a JBI compliant ESB to get started with ServiceMix is probably the easiest.