I was asked to find an open source load/performance testing tool we can use at my employer, and I've done some digging. OpenSTA seems to be our best option, although it can be quirky and sometimes unreliable with HTTPS. However, I did find something called WebLOAD which looked promising. However, it says it doesn't support HTTPS at all. Given that this is advertised as "open source", I figured perhaps it could be made to support it (even though this sounded like a strange idea...a web load testing tool that didn't support HTTPS).
Apparently the company who makes WebLOAD is a bit unclear as to what "opens source" means...check out this blog entry to see what I mean.
The search continues, but it looks like I am stuck with OpenSTA.
EDIT: GWB isn't letting me comment on my own blog (again) so I thought I'd thank the person who posted the alternative of JMeter as well as confirming that WebLOAD won't do what I need.