There is a new technique I found the other day and couldn't stop myself from trying it: Acid Wash.
For this technique you need two or three polishes of different colors. You should apply the polishes one after the other and finally remove some coats using an ear cleaner.
Products I have used:
I have made it a little differently. I have applied the second layer before the first one has dried completely and by pressing the brush harder, I created this design.
It's a very easy technique, perfect if you don't want to wear a simple color on your nails, but are too lazy to do a more complicated design :-)