NServiceBus is growing in popularity. It is open source as well. Here is a Hanselminutes episode with Scott Hanselman talking with Udi Dahan about NServiceBus to help grok it. You should definitely evaluate using it.
UPDATE: There’s also a DNR TV episode which shows what it’s like to build an NServiceBus solution from scratch here: http://www.dnrtv.com/default.aspx?showNum=199