A sudoku solver in Scratch. Yes, I know, Scratch.
Okay - technically, it's not actually pure Scractch, it's Snap, a fork of Scracth by Berkeley. Upload the xml file there and you'll be able to see the code within the Scratch editor.
This only exists because I was in a program where the instructor insisted on completing a project with a scratch derivative, so I did what I could with it.