The guys at Devver recently did a very nice writeup of the Rails Performance tools available in the market today. Devver is a Boulder based startup focused on developing great cloud-based tools for Ruby Hackers. Here is what Dan at The Devver Blog had to say:

Recently a few different Rail performance tools / web services have been released. They are leaps and bounds better than just watching your developer log and trying to fix performance issues, or just tracking down slow MySQL queries. I went ahead and installed and worked with a few of these tools and wrote a bit about my thoughts and experience with each of them.

He then gives a brief but well written overview of each of four of the existing Rails Performance Management tools. Take a look here