Mémoires is the best way to keep your journal on Mac OS X. Choose a day and write as many notes as you like. Then browse your journal by day using calendar, or search for notes using the instant-search feature.
Mémoires has a gorgeous user interface. Even though it's a powerful application, it is very easy to use. No distractions, no complicated forms: just two buttons and one search field. Even easier than a paper journal!
Mémoires supports AES-128 encryption, so your diaries will be safe from prying eyes.