If you are looking for something that also tests the functional behavior of web applications and is able to simulate more complex user interactions then you may want to take a look at the tclwebtest project. It provides a tcl api for writing tests on web apps.

It is far from being finished, but already usable to do some basic testing on ACS-like sites. I want to add some simple stress testing capabilities soon and would appreciate suggestions on how to do that. Although - if you need something that works _right now_ then it is propably not an alternative.