We launched http://blitz.io (a Sinatra app on Heroku) a few weeks back to help with instant load testing of cloud apps. We've been using this against our rails app deployed on Heroku to help us understand the performance of our controllers, CouchDB queries as well as the number of dynos.
At this point, we are really looking for feedback from developers to see if this helps you. We've also released a Ruby Gem that uses the RESTful API of blitz.io so you can instantly load test as part of your Rake tasks, post-commit hooks or any of your unit testing frameworks.
Blog: http://bit.ly/i0etWX Code: https://github.com/pcapr/blitz Gem: RubyGems.org | your community gem host
Thanks and feedback much appreciated!
K.