Possibly favorite visual effect is the slow bucket fill. The recent Terra Nil trailer shows fire behaving in just this way. There's a point in the grass where the fire starts and it radiates out slowly and evenly in all directions.
This gets really neat when there are some edges in the filled space. At the corners between spaces you get some neat effects. In the way it spreads it almost looks like a car turning around — but also it looks nothing like a car turning around, because it's just flowing continuously onward. It's fun to watch.
I remember a game I played in childhood had a painting not-even-a-minigame that used this effect with its bucket fill tool. I don't recall the name, but there was a rabbit. In retrospect it seems strange — there was no goal of paint this or paint that, there was no puzzle or paint X amount to unlock this new area. It was just an easel and some paint tools. No someplace else to get to — just a bunch of delightful tools to play with.