Just found out that Zeit.de has an online version of this awesome game: http://sudoku.zeit.de/sudoku/kunden/die_zeit/
Every day there are 3 new games (leicht = ez, mittel = medium, schwierig = difficult) you can play, the goal is to fill up all fields with numbers from 1-9 so you have the numbers 1-9 in each column, each row and each 3x3 box at the same time. Click on a field and just enter the number with your keyboard. Hit 'Spiel überprüfen' to verify your game, a small red (wrong) or green (right) text will appear in the top right corner.

Enjoy!