Siege (http://www.joedog.org/JoeDog/Siege) is awesome. I got it to handle multiple session cookies simultaneously across multiple processes. It is going to save me a lot of time and effort load testing my site. I just wish it outputted significantly more granular data so that I could run my own analysis on performance and latency across a variety of dimensions.
Regardless, it is a sweet tool. Thank you Jeffrey Fulmer (author of Siege)!