Testing tool

[Wang, Jianming]

Hi, everybody.

We have a Web application beta version.  We want to find tools to test how
many users can concurrently use this application, when the application
performance slow down while users' amount increasing, etc..  In a word, we
need a tool to do loading test.  Certainly, at current stage, we don't want
to spend money on testing tools.  Does anybody know where we can find such
tools for free?   
Thanks in advance.

Jianming Wang