If you're looking for a simple but elegant way to keep a journal then Day One is an excellent choice. Day One makes it easy to quickly enter your thoughts and memories and then sync them to the cloud.
You can use Day One on iPhone and iPad too and use iCloud or Dropbox to syn it with your Mac. Day One is well designed, easy to use and easy to access from the Menu Bar. There's a reminder system, calendar view and other handy tools to ensure your thoughts are recorded. Most recently, Day One has been updated for Retina Macs to take advantage of the new displays so it looks great too.
In addition, Day One allows you to password protect your apps, export them for editing in Word, search your entries for keywords and edit in Full-Screen Mode. It should be noted that the password feature isn't particularly secure though - the password is not encrypted and if someone were to export your diary, they would be able to read it in any application that opens TXT files. Location and weather data captured from your iOS device can also be added to Day One and this feature will be available on Mac soon.
Day One is overall a very slick and well designed journal that makes keeping a diary on your Mac a pleasure.
Download Day One 1.7.2 in Softonic