PerformanceEngineer.com

TestMaker

TestMaker is a framework and utility that builds intelligent test agents which implement users interactions in several environments. This program is developed in Java (so it is multi-platform) and it is available as open source.

http://www.pushtotest.com/ptt

I haven't played around with it much, but it's on my to-do list. From browsing the web site and seeing other reviews on the web, it looks like it may be worth a look as quick load test tool when you're in a pinch.