Here are the the forerunners among open source ESBs (in no particular order):
JBossJBoss generally has mature components in its GA releases with no vendor-lockin characteristics. Their ESB leverages JEMS technologies like the JBoss business rules engine for content-based routing and messaging. Content-based routing on the JBoss ESB can use Drools or XPath. The JBoss ESB supports XSLT and the Smooks transformation engine for XML and non-XML data formats. JBoss' ESB also runs on the JBoss application server and features a pluggable architecture for swapping out ESB subsystems.
ApacheApache ServiceMix 4 is OSGi based and a great option for integrating with an XML standards focussed landscape. Apache ServiceMix makes it very easy to hot-deploy new integration flows. Even the pluggable integration components are hot deployable. ServiceMix uses a JBI standard which provides a lot of components like JMS, BPEL, Web service, and Camel. The inclusion of Camel is a strong point for ServiceMix along with the Spring Framework, which is also supported. FUSE ESB is another great distribution of Apache ServiceMix.
Sun(Oracle)OpenESB has an easy learning curve due to its solid integration with the GlassFish Application Server and Sun's popular IDE, NetBeans. The Netbeans IDE provides countless integrated functions for administration and development. The best thing about OpenESB is its toolset. OpenESB's tools include WSDL and schema editors, a JPI manager integrated into the service manager, and Ant running in the background. Another tool is the Composite Application Service Assembly (CASA) editor, which gives you a graphical overview of integration applications. Many Java developers will love OpenESB because it comes straight from the home of Java. OpenESB is also OSGi based.
MuleSoftMule is the most used open source integration platform. MuleESB's low cost along with easy configuration, expansion, and flexibility make it very popular. Java developers will find MuleESB easy to work with because it is Java centric. There’s also a powerful set of XML schemas in MuleESB. The creation of integration flows is very straightforward. MuleESB can have fairly complex integration flows up and running in minutes. It has many connectivity, routing, and transformation options right out of the box.
WSO2Other ESB products take a relatively heavyweight approach by using the JBI specification, but the relative newcomer, WSO2, takes a lightweight approach in its ESB. It does this by focusing on Web service standards for integration. The WSO2 ESB uses Apache Synapse, a nimble Web service mediation and routing engine that focuses on providing fast XML message processing. WSO2 takes advantage of Synapse's non-blocking http://s transport implementation over the Apache HttpComponents/NIO module. This allows the WSO2 ESB to handle thousands of parallel requests using a small amount of resources and threads. You can always expect great XML support from the WSO2 ESB because well-known XML expert James Clark is a company director at WSO2.