When a square turns off, it can't turn on in the very next iteration. Squares in this in-between state are colored red. This set of rules is known as Brian's Brain. Click Start to see what happens to a random configuration of squares. In fact, almost everything ends up as a glider.
Glider guns are also very common, and they themselves are gliders.
And if you watch carefully, you might even find a glider-gun gun. Go to the previous page or the next page or the contents page.