jrnl is a great little text-based journaling tool with a command line interface. Why plain text files? I love this tidbit from the readme:
you can put them into a Dropbox folder for instant syncing and you can be assured that your journal will still be readable in 2050, when all your fancy iPad journal applications will long be forgotten.
At first blush, the interface looks really well thought out. I don't journal much, but jrnl just might get me started.