I used php curl extension multi_ functions that are only available since PHP5. The script is very simple. It accepts an integer parameter with max clients and a string parameter where a URL list file resides. You can run the script either from the command line or a browser. The script outputs average client response time, average responses per minute, total amount of responses and time elapsed during test loading. After a client responds the tool requests another URL. You can only stop the tool by killing the process.

You can try my php test loading tool yourself. If you are looking for something more serious try apache ab under Linux or Cygwin.