SharePoint Capacity Testing Tool
Posted by ken zheng on May 23, 2011
Here is the post from Moss List, I think it is a good reference for someone want to do the load testing on SharePoint.
Visual Studio 2008 Ultimate or Visual Studio 2010 Ultimate can do Load Testing/Web Performance Tests (500 and 250 simulated users respectively).
The SharePoint Administration toolkit has a Load Testing Kit in it also:
You can also find some SharePoint Performance Tests on CodePlex, and there are a few others if you look.