We're considering a messaging platform to integrate two core systems in a banking environment. We're looking at open source options. Which products have you used and can you share experiences?
If you're on Microsoft, I would start your search with MSMQ. I've used MSMQ at work quite a bit and never really had any serious problems with it. WCF has great support for MSMQ if you're using .NET.
I've also used Apache ActiveMQ for Java JMS, and had mixed results. It has some really nice tools that come with it (web-console, monitoring tools, mulitple APIs, etc.), but we've had stability problems with it. It could very well be because of our code, but it's been flaky for us.