This is a great effect for a menu, with lots of easy customization. It’s called LavaLamp. (jQuery Plugin)
I wonder if it could be made into a vertical menu. I would use it on the top left of this page.
Sometimes, I marvel at the creativeness of some people. This effect isn’t something I could come up with on my own, but it works perfectly. I feel like the guy in Amadeus who is cursed with the ability to recognize genius, but not create it himself.