The UltraESB is the latest ESB to hit the market. Its available free of charge for unlimited perpetual use, and its source code is available for partners.
The complete download is just 25MB, and that even includes optional libraries for JMS, Hessian etc, so that the user can just run all of the samples without downloading or configuring anything else. The AdroitLogic website contains many samples ranging from REST, Mock Services, SOAP, AS2/EDI, Hessian, and will soon include samples that shows JTA transaction use, load balancing and fail-over and many other options.
It also includes its own custom WS-Security library, to overcome issues seen with the use of existing options for WS-Security. By being the first ESB to support Zero-Copy proxying of HTTP/S, the UltraESB is ready to break current records in performance benchmarking.
Read about the UltraESB at http://adroitlogic.org