ServiceMix is lightweight and easily embeddable, has integrated Spring support and can be run at the edge of the network (inside a client or server), as a standalone ESB provider or as a service within another ESB. You can use ServiceMix in Java SE or a Java EE application server.
ServiceMix uses ActiveMQ to provide remoting, clustering, reliability and distributed failover.
ServiceMix is completely integrated into Apache Geronimo, which allows you to deploy JBI components and services directly into Geronimo. ServiceMix is being JBI certified as part of the Geronimo project.
Other J2EE application servers ServiceMix has been integrated with include JBoss, JOnAS with more to follow.