Puzzle by Nikoli W Slitherlink (Windows)

In this - one of Nikoli's most awesome original puzzles - you connect the dots to make a single ring. "Slitherlink" is a puzzle in which you connect dots with a line according to the rules. The greatest attraction is the fact that there exist an unlimited number of set sequences known as "theorems". With this puzzle, you can actually feel your own skill level rising with each theorem you learn.