You can change the square's color, move it around the desktop, make it permanently visible, or make it transparent. But the most interesting feature, which you'll always be using it for, lets you write down notes so you don't forget anything important.
Metro Pad is a very small, lightweight, and easy-to-use application that doesn't require installation. Just unzip the contents of the ZIP file to a folder and start writing down anything you want. The changes will be saved automatically every time you close the program.