We are using NewRelic, they are growing as the leading company web Application Performance Management (APM).

We are use it to monitor, troubleshoot and tune our web apps and sites. We have thousands of pages and daily users.

After just a few months we had identified all of our bottlenecks and manage to increase our speed by a factor of 2.5.
It can solve performance issues for Ruby, Java, .NET, PHP and Python apps.

You can check a description of this apps together with user reviews in the NewRelic's listing at GetApp.com.