JBoss ESB is JBoss' open source Enterprise Service Bus.

One of the main goals of JBoss ESB is to enable you to knit together disparate systems. JBoss ESB does this by abstracting the differences between these systems by treating each of them as a logical service on the ESB. The services communicate via messages, as on the bus, everything is either a message or a service. As a result, regardless of the inner workings/architecture of any service, you can "plug" them into the ESB and connect them without having to write system-specific "glue code".

JBoss ESB does not dictate to you specific integration patterns for any types of legacy systems. They are all treated as logical services on the ESB. This approach ensures that JBoss ESB itself does not become one more legacy system that you have to compensate for by writing integration "glue code".