In a Slitherlink puzzle your goal is to form a single loop through the game field using numerical clues.
You can read more about history and rules in Wikipedia http://en.wikipedia.org/wiki/Slitherlink
In addition to classic square grids of different sizes and difficulties this implementation of Slitherlink offers hexagon, pentagon and mixed grids. Try solving those, it may require different ways of thinking than with squares.
Automatic coloring of clues and lines helps thinking, but can be turned off for more classic look and tougher challenge.