Being a geek, Lava Lamp live wallpaper is one of my all time favorites. It uses actual heat physics to attempt a lava lamp simulation. There is a "heat gradient" (hotter at the bottom, cooler at the top) that causes the blobs to heat up and start to float when they are at the bottom of the screen. When they're on top, it's cool enough that they start to cool and sink back down.
I also like the fact you can create some color combos that you couldn't get with a real lava lamp. Or, you can set it up to be a new color whenever you turn your screen on.
Check it out.