Slitherlink (BrainBashers)

Slitherlink is a loop-drawing puzzle. The board is a lattice of dots, and some of the little squares between the dots hold a number from 0 to 3. You join neighbouring dots with lines to build one continuous loop. Each number tells you how many of its square's four sides are part of the loop, and squares without a number can have any count. The loop may never cross or touch itself, so every dot has either no line or exactly two lines passing through it.

BrainBashers, Kevin Stone's puzzle site, posts 5x5, 7x7, 10x10 and 15x15 puzzles daily, each at Easy, Medium and Very Hard, with a 20x20 weekly special. The play page greys out finished clues automatically (you can turn that off), can highlight clues, and has check, answer, auto, print and restart buttons, an option to show mistakes, an optional clock and an alternative dot style. A help page with a walkthrough covers the standard patterns.

The set changes at midnight GMT or at your chosen time zone, and the date list keeps a few weeks of earlier days. No adverts, no account.

It suits players who like a puzzle that builds from small local patterns into one big shape, and the Very Hard grids are a real test.