Notational Velocity is great. Just point it to a dropbox folder and set it to use plain text files and you're set with syncing. There's even an equivalent program for Windows - ResophNotes http://www.resoph.com/ResophNotes/Welcome.html.
Right, plaintext in dropbox? <tinfoil_hat> what about local encryption? saving personal notes in plaintext files...</tinfoil_hat>
But it is so convenient and simple to use!
It does in fact support encryption. I sync my data across dropbox as well.