As you can see by the answers to this question, there are lots of programs available for load testing. In my experience I have found Siege to be my personal favorite. Its simple, provides most of the options needed to do load testing. Here is an example usage:
seige -c 10 -t 1M http://www.google.com
This will send 10 concurrent requests to google.com for 1 minute. If you want to test more than one page you can also provide a file which contains a list of urls.
Note that a tool like seige is appropriate when you have a single or set of urls that you would like to test. If you want to test more complicated flows (e.g. user logs in, submits entry, logs out) then you will want to use something like selenium.
