boomerang is a piece of javascript that you add to your web pages, where it measures the performance of your website from your end user’s point of view. It has the ability to send this data back to your server for further analysis.

With boomerang, you find out exactly how fast your users think your site is. boomerang comes to you from the Exceptional Performance team at Yahoo!, aided by the Yahoo! Developer Network. boomerang is opensource and released under the BSD license, and they have a whole bunch of documentation about it.

Requirements: -
Demo: http://yahoo.github.com/boomerang/doc/
License: BSD License