I've written about quite a few note-taking apps in my pursuit of the best one. Most shy away from the Post-It note paradigm because OS X comes with Stickies, a pretty decent representation of the repositionable note, but Memo takes it head-on.
Memo is basically Stickies with iCloud and password support. Create a new little yellow note, write yourself a reminder, text snippet or love-letter to your Mac, and move it to where you need it. Like many other programs, the note sits behind the active window and can be easily resized to accommodate more text.
The notes can be password protected by clicking the little padlock icon, which keeps prying eyes out of your business if you're letting someone else use your Mac. You can change the font used, size of text and all the standard paragraph formatting you might expect. Unfortunately you can't change the colour of the note itself -- it's yellow for offline memos, blue for cloud memos and green for private memos, which are secured with a password.
The blue "Cloud Memos" sync with iCloud to any Mac or iDevice with your iCloud ID and the free Memo app for the iPhone or iPad. If you're looking for a Stickies replacement for just text, Memo has a couple of advantages that might make it worth looking at. It's free, available in the Mac App Store and has free companion apps for the iPhone and iPad.