apachebench is an excellent performance and load-testing tool for any website, and Drupal-based sites are no exception. A lot of Drupal sites, though, need to be measured not only under heavy anonymous traffic load (users who aren't logged in), but also under heavy authenticated-user load.
Drupal.org has some good tips for ab testing, but the details for using ab's '-C' option (notice the capital C... C is for Cookie) are lacking. Basically, if you pass the -C option with a valid session ID/cookie, Drupal will send ab the page as if ab were authenticated.
Instead of constantly going into the database and looking up session IDs and such nonsense, I have a simple script, which is quite revised from the 2008-era script originally from 2bits that worked with Drupal 5, which will give you the proper ab commands for stress-testing your Drupal site under authenticated user load. Simply copy the attached script (source pasted below) to your site's docroot, and run the command from the command line as follows: