Load testing Web Applications and Client Side Javascript

Two topics here.
 load testing on the server
 load testing on the front end

Load testing on the Server.
http://jmeter.apache.org/
http://grinder.sourceforge.net/

This is kind of in the middle.
For load testing the server, verifying responses, and interacting with the page returned. Multi-mechanize looks pretty sweet. Its a python library that reminds me of selenium somewhat.
http://testutils.org/multi-mechanize/scripts.html

Load testing on the front end

Jiffy is an end-to-end real-world web page instrumentation and measurement suite. Jiffy is a novel idea in load testing tools instead of measuring the performance of the web server. We are measuring the time it takes to load the web page on the client and run the javascript.
http://code.google.com/p/jiffy-web/

This would probably be a nice setup at a company if you had a great set of selenium tests that were maintained. This would most likely take a while to run and by running them in parallel you could definitely speed up the process.
http://selenium-grid.seleniumhq.org/
http://selenium-grid.seleniumhq.org/setting_up_selenium_grid_on_ec2.html

Selenium RC is a project for language bindings to selenium. http://seleniumhq.org/projects/remote-control/

If you have money to throw at the problem, this may be of interest. http://www.crunchbase.com/company/browsermob