They might be primarily a west coast thing, although I sure think I've seen them elsewhere...
They're little plates - I'm guessing about a foot in diameter - that kinda extend upward a bit as well, but compress down when stepped on. You put them at 25 yard intervals, and they're marked as such (ie 75, 100, 125, 150, 175, 200, 225, 250). Typically courses put them always on the same side of the fairway, so you always know where to look. They work really well - you're never that far from one so you can't either eyeball it or step it off if you're feeling anal - and they really don't look bad at all.
TH