[icambron]

Which also shows the evolution of the yellow line idea: another line (usually blue) for the line of scrimmage, which turns out to be really useful. I think I've also seen a line for maximum field goal range.

[Someone]

And you probably also have seen advertisements inserted behind the end zone. I do not know how much use they make of it, but that allows the broadcaster to send different advertisements to different markets (the low tech variant that I have seen of this in soccer is to have one country show the main overview from one side of the field, and another country the main overview from the other side. That way, one can sell the main advertising space at the center of the field across from the main camera twice)

Also, in speed skating, the technology gets used to project a moving line indicating where a skater should be to reach some goal (win the race, world record, etc)

[baddox]

The concept was used a lot in the Olympics. I remember seeing national flags superimposed in swimmers' lanes, and they also used pace lines for swimming.