The default sticky notes app for OS X is great, but you can't use it for any type of confidential information you might want to keep at the ready. Memo is a simple app that allows you to password protect and hide your notes.
You can manually lock your memos as needed, or to auto-lock after one minute and you can set individual passwords for each note. Memo doesn't do anything else, but if you love using the sticky notes on your Mac but want to hide the occasional bad haiku or gift list, Memo does its job and ensures nobody else can see them. It's a free download on the Mac App Store.