You should also take a look at FiveRuns' TuneUp. It is completely free, installs as a plugin into your Rails application. Keep in mind that it does send data back to their server (mostly very harmless information), but that might bother some people (namely government agencies would be very unhappy about that). I think you'll be surprised how great the tool is considering it is free.
Another nice feature of it is that you can post your run data for others to comment and help you diagnose your long queries, or bad code.
