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Re:Post mortem of a failed HackerNews launch by pearkes on Thursday November 29, 2012 @11:42AM (#4848612)
Blitz is also worth a mention.

http://www.blitz.io/

Re:Post mortem of a failed HackerNews launch by ohashi on Thursday November 29, 2012 @12:24PM (#4848900)
Can also connect newrelic with it and get stack traces logging how much time each component of your app is taking including DB queries.
Re:Post mortem of a failed HackerNews launch by druiid on Friday November 30, 2012 @01:59AM (#4852184)
Blitz is good, but I've found the way to build test-cases fairly limiting. I've used http://www.loadimpact.com to good success.

Addendum: Oh, load impact is pricey though.