Juju is a Sudoku program written in Java. Its source code is released as public domain and can be found here.
The objective of Sudoku is to fill in the entire grid with numbers which follow a simple set of rules. Every row must contain each number 1-9 exactly once. So must every column, as must every 3x3 square.
Use the mouse or arrow keys to navigate the puzzle. Press the number keys to fill numbers in. Delete or 0 clears a number. Press H for hints; places on the grid which can be determined with the numbers currently present will be highlighted. Press F to automatically fill in the current hints. Press T to toggle whether erroneous numbers are highlighted red when selected. Press N to begin a new game.
Java security settings are fickle - if your browser won't run the applet, find the Java Control Panel ("Configure Java" in the Windows start menu), go to the Security tab, and either lower the security setting to medium, or add http://pineapplemachine.com to the whitelist.