Tested on Firefox. Untested on other browsers. :P
Couple of hours ago something crazy came to my head. I want to control the flow of time by manipulating the Date() constructor and see what's gonna happen.
The setInterval and setTimeout function is also modified to be slower.
As a live example, I have included the script on this page too. All animations on this page will be 4 times slower! Compare it with the home page!
You can also have the slow down bookmarklet.
Try it! Drag the bookmarklet to the bookmarks bar, and then go to some site with animation effects (for example, http://jquery.com/) and activate it. You will see that all the animations in the page will be 4x slower.
This bookmarklet will not work with CSS transitions.
I'll see if I can do something fun with this.