Was thinking about this topic today, and I remembered a few years back I used Microsoft's Web Application Stress Tool. It did the job (simple stuff, nothing terribly complicated), and it was free, if sometimes a little difficult to use properly. Apparently it's not maintained anymore, since the listed version is still compatible only with W2K.

Now, I was any apps out there that people really like for this job, on any platform? What do you use/recommend for web apps stress testing?