Sudoku4ever Plus (Android) app for $0.99

Sudoku4ever by 1gravity offers an unlimited number of puzzles, eight difficulty levels from Very Easy to Nightmare, a world-class hint engine, game statistics and global high scores.

A standard Sudoku puzzle consists of a 9x9 grid, with a total of 81 cells. Each position in the grid is called a cell. A cell can be empty, or filled with a digit. This digit can be any digit between one and nine. You must place digits into the grid in such a way that every row, every column, and every 3x3 box contains each of the digits one through nine.