jmeter

JMeter - I like it for load testing sites...

get:
http://jakarta.apache.org/jmeter/

how to use it:
http://www.onjava.com/pub/a/onjava/2003/01/15/jmeter.html

review of it:
http://www.linux.com/feature/34432

or if you want command line tools:
http://www.linux.com/feature/143896