Fresh out of of the oven:- ESB Guidance
Microsoft's ESB Guidance for BizTalk R2 formally ESB Toolkit, is finally released key features includes consuming lots of features of BTS away from BTS
like consuming the mapping/transform features
an orchesteration that performs itinerary. where you can process message accross multiple endpoints one by one
additionally (and the nicest) statless message routing (just about time) basiclly if you are building a messaging system where you need to quicly route messages BTS was not the solution as BTS presist the messages in SQL DB (named MsgBox) . you probably use WCF routing with lots of channel/channel factory code.
a very nice error handling across your service bus centerlized in a way
in total, comping ESB guidance with WCF services + BAM interceptors should give you a good head start when building large scale messaging system
http://www.microsoft.com/downloads/details.aspx?familyid=e3957253-24ce-45aa-ac32-60abffe15bac&displaylang=en
all in all writing applications & services just got even wilder :)
Kal