Check out Apache JMeter for load testing.
On its website:
Apache JMeter can be used to test performance on both static and dynamic resources (files, servlets, Perl scripts, Java objects, databases and queries, FTP servers, etc.). It can be used to simulate a large load on a server, network or object, to test its strength or to analyze the overall performance for different types of load. You can use it to graphically analyze performance or to test the behavior of your server / script / object under heavy parallel load.
I used it at work before to test the load, and it works great.