These are useful for diagramming any message based system - I'm very keen on UML but it does not really address MOM systems in the large. Below is a Gregor-gram showing how a Sales Order Process might integrate SFdC, Zuora (Z-Billing) and Oracle Financials (a pattern that most businesses will need):
This diagram allows us to show how message move through the system and are transformed. There's also another good book is Chappell's Enterprise Service Bus which is focused on the ESB implementation of MOM (no surprise given the name!).
It has another diagramming techique, which at the moment I'm using for creating a static view of the ESB:
This is closer to a static UML view (deployment maybe), but it also allows you to express how the components connecting to an ESB exchange messages (HTTP/WCF etc). I feel that Gregor-grams are better for the dynamic view. The book never names these diagrams, so 've taken to calling them "Chappell-grams" given that they are meant to be an evolution of Gregor-grams.
Using Gregor-grams and Chappell-grams together allows us to document how a system works from both a dynamic and static view.