Super Mario Cross Over is now ever so slightly improved including the addition of Ryu from Ninja Gaiden as well as a new NES sound emulator. The creator posted a full Changelog for your edification.
To play, simply bring up an Excel report and place it on your desktop. Then turn down your speakers or connect headphones and open this website in a separate window. Then, when your boss comes by, pause the game and swap back to the Excel spreadsheet. Repeat until 5pm.