Daily Quadruple Sudoku
The extra rule
A circle names all four digits in the cells touching it.
Each circle sits where four cells meet and lists every digit in those four, in no particular order. A circle reading 1 1 4 9 means those four cells hold two 1s, a 4 and a 9, and you have to work out which goes where.
Repeats are the useful part. Two cells in a circle can only share a digit if they are in different rows, different columns and different boxes, which pins them to a diagonal pair straight away.
These grids start with almost no digits, often just one. The circles carry nearly all of it.
Controls
Click a cell and type a digit, or use the pad. Space toggles pencil marks. Arrow keys move. Progress saves in this browser as you go, so you can come back to the same puzzle later today.
Solvable Without Guessing
Every puzzle here was generated and then handed to a solver that is only allowed techniques a person can use. If it could not finish the grid without a guess, the puzzle was thrown away rather than published. That also makes the answer unique, because every digit it placed was forced.
More to Play
There is also a kropki sudoku, a xv sudoku, a greater than sudoku, a nonogram with hand drawn pictures, and 2048. All of them are on the games page, and the solvers for real objects are on the puzzle tools page.