Scrutinizer was originally released in 2007 as a commercial tool. It is now available under an open source license.
It simulates how people actually see web pages at a 100 millisecond level by emulating the high resolution foveal view, and lower resolution and (relatively) color blind peripheral vision within a web browser application built in Adobe Air (Flash’s big brother).
Check out the latest at https://github.com/andyed/scrutinizer.
Related: Launch Announcement.