Iona has released Celtix Enterprise
, their open-source ESB. Celtix provides a lightweight services creation framework, advanced messaging through Apache Qpid, and flexible deployment through Tomcat, Spring, or ServiceMix. Celtix is built with code donated to Apache by Iona, along with other contributions such as XFire from Codehaus.
An ESB itself is fairly complicated to explain. An ESB refers to a software architecture construct, implemented by technologies found in a category of middleware infrastructure products usually based on standards, that provides foundational services for more complex architectures via an event-driven and standards-based messaging engine (the bus). As such, it's only one part - although a crucial one - in a service-oriented architecture.
Celtix also comes with the SOA Tools Platform, an Eclipse plugin. STP will be able to installed via the Eclipse update mechanism eventually; it can currently be manually installed with the provided .zip files. (As this is a new release, some minor glitches in documentation and online references can be expected.)
With registration, Iona is offering 30 days of support for Celtix. The download of Celtix itself is free.