Pure Sudoku Free Edition
If you've somehow missed the sudoku craze, here's how it works: It's a number puzzle in which you fill in the blanks on a nine-by-nine grid so that every row and column contains the numbers one through nine, and every three-by-three square contains the numbers one through nine. No number can be repeated in any line, row, or three-by-three square. The game starts with a group of numbers already filled in, and you have to fill in the rest.
Many newspapers have daily or weekly Sudoku puzzles, and there are numerous books of them as well. This program is a way to get games on your PC, and for free. Start the program, then choose your level of difficulty, and you're ready to go. Filling out the puzzle is simple: Click a number, then click the square where you want the number to go. If you put in a number that is already found on the same row, column, or three-by-three square, you'll be told it's already there.
You'll need to know one very important piece of information before using this game: In order to put multiple possible numbers in any square, right-click, instead of clicking. That way, you can narrow down the possibilities as the game proceeds. It's almost impossible to solve Sudoku puzzles without this ability, so make the right-click your friend.
There's also a for-pay version of this game with extras such as being able to save games, and that offers tips. But you don't need to pay to have fun--the free version is very good as well.