The lastest version of WebSphere MQ aka MQ Series is 7
MQ home page: http://www.ibm.com/software/integration/wmq/
Download it here: http://www.ibm.com/software/integration/wmq/freetrial/
The info center (documentation): http://publib.boulder.ibm.com/infocenter/wmqv7/v7r0/index.jsp
The info center for WebShphere MQ Series Version 6: http://publib.boulder.ibm.com/infocenter/wmqv6/v6r0/index.jsp

WebSphere MQ Series performance tuning recommendations:
http://www.ibm.com/developerworks/websphere/library/techarticles/0712_dunn/0712_dunn.html

Harware alternative to WebSphere MQ Series: http://mintsl.com/Solace-JMS-HW.php