It called turn.js and the site demos the plugin, with a good reference guide too.
In a recent interview Matt Cutts said that your sites loading speed will be a huge factor towards the way google ranks it. Google and many people at google believe the web/internet should be fast and so they should reward sites that load quicker and are optimised through reuse of code and parrallel loading.
According to Cutts, speed hasn’t played a role in rankings in the past, but that may very well change. Watch the video for more details about Caffeine and Google’s resources for helping webmasters improve site speed.
So tools such as Yslow! and firebug will show you a waterfall chart of how you page loads and what is holding it back. These can be used to improve the speed and ensure that pages are cached more efficiently.