At the risk of being accused of shameless self promotion I would like to point out that in my quest for a free load testing tool I went to this article: http://www.devcurry.com/2010/07/10-free-tools-to-loadstress-test-your.html
Either I couldn't get the throughput I wanted, or I couldn't get the flexibility I wanted. AND I wanted to easily aggregate the results of multiple load test generation hosts in post test analysis.
I tried out every tool on the list and to my frustration found that none of them quite did what I wanted to be able to do. So I built one and am sharing it.
Here it is: http://sourceforge.net/projects/loadmonger
PS: No snide comments on the name from folks who are familiar with urban slang. I wasn't but am slightly more worldly now.