test load internet web app

HI
I have an online web app which I
want to test it load.which tool you recommend? I want to simulate 15K
cuncurrent users. can I test this load my self using another server with
 open source tools to send trafic to the web app or do I need to
contract an online service? <br>
if yes which one you recommend ?

thanks

marco

------------------------------------------------------------------------
Securing Apache Web Server with thawte Digital Certificate
In this guide we examine the importance of Apache-SSL and who needs an SSL certificate.  We look at how SSL works, how it benefits your company and how your customers can tell if a site is secure. You will find out how to test, purchase, install and use a thawte Digital Certificate on your Apache web server. Throughout, best practices for set-up are highlighted to help you ensure efficient ongoing management of your encryption keys and digital certificates.

http://www.dinclinx.com/Redirect.aspx?36;4175;25;1371;0;5;946;e13b6be442f727d1
------------------------------------------------------------------------