BizUnit is being used widely for testing BizTalk solutions
What is BizUnit?
BizUnit is a Framework for Automated Testing of Distributed Systems.
It is a test framework that is intended to test Biztalk solutions. Actually BizUnit has quite a significant number of steps that have nothing to do with Biztalk per se and can be used for any other integration product as well.
BizUnit currently takes a black box approach to test the Biztalk solution and is based on "external observable behavior".
For more info about BizUnit - http://www.codeplex.com/bizunit