Hi,
Microsoft used to provide a free tool call MS Web Application Stress, however it would appear that they have withdrawn it from download. Fortunately I managed to find a link to download it. It can simulate multiple clients and threads to the ACE. I've been using it recently to test various conditions on my ACE. It works well for a free tool.
If you want something more fancy and don't mind paying, you could try NEOTYS - NeoLoad, which I've also used in the past.
Here is the link for the free MS Web Application Stress:
http://stackoverflow.com/questions/3326922/where-can-i-download-microsoft-web-application-stress-tool
Hope that helps.
Dom