NServiceBus and MassTransit are two tools that can be used to implement messaging with MSMQ and other message queues.
I find that once you start using messaging to have applications talk to each other, you don't really want to go back to the old RPC style.
My question is, what other tools are out there? What tools do you use?
Rogue Wave is very popular [ http://roguewave.com/products/hydra/ ]
So are IBM's Websphere offerings [ http://en.wikipedia.org/wiki/Mqseries ]
Apache ActiveMQ is probably the most popular and powerful open source message broker out there with the most active open source community behind it as well as commercial support, training and tooling if you need it.
One of the more interesting aspects of ActiveMQ is its wide support for a large number of different language bindings and transport protocols
WCF is extremely powerful and should be looked into by anyone in the .NET space starting up a message based system.
I would recommend against BizTalk unless you can make a lot of use out of it's adapters (ie. you have a lot of old systems to communicate with).
Nuedesic makes a great WCF based ESB, Neuron, if you are willing to pay a bit.
We use WCF services for synchronous message based operations, and nServiceBus for anything asynchronous.
I use IBM software stack because it has the widest set of features (pub/sub, async, sync) and platform support (60+ combination of platform, languages) and also a great set of free tools provided by IBM
For Operations, I use use the linear log rotation IBM WebSphere MQ supportpac
For development and testing, I like RFHUTIL to generate fake cobol, java, MS objects, other binary and text objects and SOAPUI to invoke HTTP web services. If I need to invoke MQ based web services, I go back to RFHUtil. Of course Websphere MQ Explorer for admin.