Iona released their latest version of Celtix Enterprise ESB as an open-source offering. I've been toying around with this product a bit. I was impressed at the initial install and configuration. It worked right out of the box which can be a bit unusual for open-source products.
The thing I liked the most is the lightweight nature of the product and the ease of configuration. In particular I liked the routing functions, content-based, fail-over etc. They have several good samples included with the product to get you started. The documentation is not bad for an open-source product although the configuration files that drive most of the functionality could use a bit more documentation.
I'm not sure I would deploy this as a main ESB solution but it might be a good addition to an existing infrastructure. Particularly where you might want to deploy some lightweight distributed components. Overall a very good effort. I'll post more on it as I dive deeper into it.