sgt-puzzles. Simon Tatham's Portable Puzzle collection.
Contains a bunch of simple puzzles, of the minesweeper and sudoku style. Loopy is my favorite.
Available for Linux, Windows, MacOS, Android, and anything with a web browser and a mouse. Packaged in Debian and F-droid, and probably many other places.
I like it for time wasting in lines at the DMV, for a low-stakes game when anxious, and for falling asleep.