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>

It does in fact support encryption. I sync my data across dropbox as well.

https://github.com/scrod/nv/wiki/Database-Security

But it is so convenient and simple to use!