iNotePad can encrypt notes and folders using Mac OS X's built-in AES-128 encryption (see the FAQ for more details):
To encrypt or decrypt a note or a folder, select it in the list, then click the "Encrypt/Decrypt" button in the toolbar.
WARNING: Remember your password when encrypting an item, as there is no way to decrypt your data without the password you used to encrypt it.
You can also encrypt the whole database each time you quit iNotePad: Select "Encrypt current database when closing" within iNotePad's Preferences > Current Database Settings.