Solo is the number puzzle most people call sudoku: fill the grid so that every row, every column and every marked block holds each number once. It comes from Simon Tatham's Portable Puzzle Collection and makes a new grid every time you ask, at several levels of difficulty, each one with a single solution.
The Type menu goes well beyond the usual 9x9. There are small 4x4 and 6x6 grids for a quick game, Jigsaw grids with irregular blocks, X grids where the diagonals count too, and Killer grids where cages must add up to a total. You can pencil in candidates, undo as far back as you like, and ask for the solution if you give up.
Controls
Click a square, then type a number. Backspace clears it. Right-click a square and type to pencil in a candidate. Needs a keyboard.
Genre
Puzzle, Logic, Numbers, Classic, Desktop
Price
Free — no account, no download, nothing to install
Tips for Solo
Start with the row, column or block that already has the most numbers.
Use pencil marks (right-click, then a number) once the easy squares are gone.
The Type menu has smaller grids and harder ones; Trivial is a good warm-up.