This demo simulates water solidifying into ice-- an interesting process I spent many hours watching as I grew up in Glenallen, Alaska.
What you see is temperature (black for very cool, to red, to yellow for very warm) and formed crystals (shades of blue depending on the crystal orientation). The domain starts at random temperature fluid, with a single ice crystal in the center. As time progresses, the fluid's heat diffuses outward to the cool border.
Cold fluid near a crystal edge will crystalize, releasing heat (the heat of fusion) into the surrounding fluid. This heat prevents nearby fluid from crystalizing, causing the characteristic elongated shape of ice crystals-- an opportunistic feeler shoots through the cold fluid, heating it; so the feeler cannot grow until it cools.
This version uses bizarre approximations to the true physics and byte array in a (misguided) attempt for better speed. If you'd like a more physical version, where the values actually have units, try this tar file.
Link to the source code.