Tools to monitor website performance?

Hi,

I'm curious what the current state-of-art for website performance monitoring is?

Most of the tools I've seen are either Pingdom-style applications, which simply tell you if a website if up or down, or they're like YSlow-style, which run a bunch of tests to see if you're following best practices.

But is there something like say, Chrome's SpeedTracer that you could run as a service, and monitor your website's performance over time? Or before/after each commit?

What would you guys recommend to track the performance of your own website over time, or gather deep-level metrics?

Could you use something like PhantomJS to achieve this?

Are there any existing open-source projects that already do this?

Cheers , Victor