Microsoft just released ESB 2.0 Toolkit. It is based on BizTalk 2009. The previous version of the ESB was called ESB Guidance 1.0. It was released in 2007.
The new version has made significant improvements in usability and feature.
You can learn more about it here.
If you are considering ESB for your enterprise you should check out the ESB 2.0 Toolkit.