Award winning IBM WebSphere MQ (WMQ) software has been having a monopoly in this space for over a decade now. WMQ has about 83% of market share (in 2005 - probably more now). The product is so popular that a lot of consultants working in space can't think beyond this product for the advantages it offers. Here is a comparison of are 47 messaging middleware products based on the cost, suitability for enterprise deployment and product maturity in market with links where you can find more information about it.
Please feel free to contact me should you need further analysis on comparing them. Let me know what are your thoughts on options in this space and which one of these is your favorite product.