There's something to be said about the sentiment behind a quickly drawn note, which is the exact concept behind Tiny Notions. Lisa Przybylski does (almost) daily post-it drawings for no reason at all. If I found any of her drawings lying around my desk at work, it'd pretty much make my day.
No one is more of a smart ass with their doodles than Marc Johns. Take a flip through his post-it note drawings for insight into the thoughts of logs, socks, antlers and paper clips.