Everyone else is pointing to Flow, so I might as well.
A while back, after seeing Spore, I wrote down in my design notebook (actually a Word doc on my machine), “You could do just the first level as a casual game.”
Lo and behold, here it is. First time I loaded it, btw, no critters appeared at all — so if you find yourself in a completely empty space, try reloading or heading for the circles that pop on the edge.