Slither Link is a nikoli puzzle sometimes called “Fences.” Slither Link puzzles consist of a field of points in a square grid with numbers inside some of the individual squares. The goal is to connect the points to form a single loop with the constraint that each number must be enclosed on as many sides as indicated by the number. There’s a nice flash tutorial for the puzzle on the nikoli site.
I bring this up because I just came across a great collection of Slither Link puzzles through Passion For Puzzles. Also, if you’re looking for an interesting variation on Slither Link, check out last year’s US Puzzle Championship test. There’s a puzzle called “False Field Fences.” Numbers inside the loop tell the correct number of enclosed sides, while numbers outside the loop indicate an incorrect number of enclosed sides. Of course, before you solve the puzzle, you don’t know which are on the inside and which are on the outside.
I recently got hooked on the puzzle Slither Link And I would like to share my collection of links for online play of the game:
http://www.puzzle-loop.com/ has a huge collection of various sizes and two difficulty levels.
http://www.pro.or.jp/~fuji/java/puzzle/numline/index-eng.html has a very nice set of puzzles. Unfortuantely there are only 38 puzzles total here.
http://www.kwontomloop.com/ – great very difficult puzzles
http://janko.at/Raetsel/Schlangenlinie/index.htmis a German site with several puzzles.
http://www.puzzle.jp/letsplay/play_slitherlink-e.html – Puzzle Japan/Nikoli. Mostly easy puzzles, good for an introduction.