Day One is the easiest and best-looking way to use a journal / diary / text-logging application for the Mac. Day One is well designed and extremely focused to encourage you to write more through quick Menu Bar entry, a Reminder system, and inspirational messages. Support for Dropbox syncing allows easy backup and syncing with the Day One iPhone application.
Note: Day One is not encrypted or password protected on this version. If this is an issue for you, please do not purchase this app yet.