Stickies - Stickies is a PC utility written to replace Post-It-style notes left stuck to your monitor.
Memo - Sticky Notes - Memo is a simple and elegant app for quickly jotting down notes on your Mac.
Indicator Stickynotes - Floating colourful sticky note windows on your desktop!
TopXNotes - TopXNotes is a personal notes and information organizing tool that you can use on iOS or Mac OS.
Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
Pystickynote - Stickynotes for your desktop easily from the command line.