I've been interested in the JMeter project for quite some time. It's an open source framework that you can use to perform stress and volume testing of web applications (in fact, it's much more flexible than that - you can also point it at databases and JEE objects).
It appears to offer some of the functionality provided by the Load Runner stress and volume testing suite (which costs big $$$), including coordinating a number of load generation servers to hit a web application, ramp up and ramp down load generation to simulate different usage patterns, attach timers to key events and (most importantly) collect and analyse the results.
To build your test cases it has a proxy server that records the http traffic between your browser and the web application server. One downside to this proxy server is the lack of support for SSL (which is almost always used in many enterprise web applications).
One thought I was having was to use Selenium to record https sessions (as it is embedded directly into firefox), it would then be a simple translation process to convert the selenium scripts into JMeter test cases.
Another thought I was having was to use stunnel to connect to the https server and use the jmeter proxy through an unencrypted local interface. Anyways, will let you know how I go with my testing...