IBM provide a free, powerful command line tool called perfharness.
Although aimed at benchmarking JMS providers, it's really good at generating (and consuming) test messages. You can use data either generated randomly or taken from a file.
The power features include sending and consuming messages at a fixed rate, using a specific number of threads, using either JMS or native MQ, etc. It generates statistics telling you exactly how fast your queue is performing (hence the name).
The only down side is that it's not super intuitive, given the number of operations it supports.