I just stumbled across this nail varnish the other day on youtube and thought it looked pretty awesome. You just put it on top of other nailr varnish and as it dries it sort of cracks. I thought it looked pretty awesome, and it was acquired. I quite like the effect, though it can be tricky to get just the right amount on before it starts to crack, and it also dries on the brush pretty quickly too.
Anyways this is how it turned out on me
And the link to the tutorial on the Barry M site
Instant Nail Effects - Make-up Tutorials - Barry M