Another interesting, and often overlooked, use for an ESB is to support Functional Tests for other development efforts. Because of its plethora of connectors and relative ease to configure, if you have a development project that relies on external services, you can set up an ESB to ‘mock’ those other services. It can emulate a (brain dead) database server, an application server, a web server, email server, bizzare application server, or whatever else you might need.
The ESB can either come up as part of your build, or be configured somewhere, and left running, to dummy whatever it is you might want to run quick build-time tests against.