In the OSS space (though I've never used them as a BizTalk replacement personally - this is anecdotal) you can use one of the Java/J2EE Messaging engines such as OpenMQ (which is the Sun enterprise one rebadged and without support). If you need Orchestration / Choreography (i.e. SOA/ESB pieces) on top of this, you could look into something like Apache Mule
