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:

Load Testing Kit (SharePoint Server 2010)

You can also find some SharePoint Performance Tests on CodePlex, and there are a few others if you look.