There are many open source and commercial ESB implementations available.
All major JEE vendors (BEA, IBM, Oracle, Sun) have an ESB in their catalog. It is unremarkably based on their middleware technologies and is usually at the core of a much broader SOA product suite. There are also some commercial ESBs such as Progress Software, IONA Technologies, Windows Azure service bus and Software AG.
Apache Camel, Apache ServiceMix, Apache Synapse, Red Hat Fuse ESB (based on Apache Camel), JBoss ESB, Spring Integration, Talend Open Studio for ESB are some of the open source frameworks.
The complete list is listed here.