We used it to load test Battlelog (the online service for Battlefield 3) and successfully simulated over 2 million concurrent users hitting the service using EC2.
See http://locust.io for more info.
(Disclaimer: I'm one of the authors)
I also recall being extremely impressed (you know, the whole.... autoscaling!!!! thing) and thought it was an internal Amazon product.
"ATTENTION: This project has several open pull requests and is in need of a maintainer! If you have experience with EC2, Python and load testing tools and would be interested in taking it over, please open a ticket telling us why and we’ll get back to you. Thanks!"
is anyone fancies it!