A sudoku with nothing printed inside the grid. Outside some row and column ends, the three digits of the first three squares from that edge are printed, in no order. Rows, columns and boxes still hold 1 to 9 exactly once.
Rim is a free daily outside sudoku from Mind Loft. Each day gives you a 9×9 grid with no digits printed inside it at all. Instead, outside some of the row and column ends, the three digits that appear in that line's first three squares reading in from the edge are printed, in no particular order, and rows, columns and 3×3 boxes still hold 1 through 9 exactly once. The gutters that are left blank are the puzzle.
There is no arithmetic anywhere: a printed triple says which three digits sit in those three squares, not where, so it reads as a naked triple you did not have to find, and two triples meeting in a box name six of its digits between them. Select a square and every gutter that speaks about it lights up. Every board has a single solution you can always reach by logic, with no guessing. Wrong entries are never flagged, so spotting your own slips is part of the puzzle, and a clean solve scores a perfect 10 with the daily leaderboard decided on the clock.
A new board drops every day at midnight Eastern; weekdays print fewer triples as the week goes on, from 30 of 36 on Monday down to 16 on Saturday, and the Sunday Edition prints thirteen. No app, no signup, play free in your browser. Want the rest of the family? Try Suds, the classic 9×9, Sando, the sandwich sudoku, Frame, the one with the sums, and Diag, the one with the diagonals.