
Mariner Software makes a program called MacJournal, which can be used to create, edit and manage journals and journal entries. And blogs, too, which can be a kind of a journal.
What's cool it's that it can post to popular online blog services, such as LiveJournal, Movable Type (the technology behind NYU Blogs), Blogger, WordPress, "and more". However, it can also be simply a journal that you keep on your computer. In fact, that is how their software was started (back in the days of Mac OS 8). You can store and manage your journal on your computer; it sounds like it might not be that interesting, to keep a journal offline, but trust me, for people who keep journals, it is a very good program. I used to use it a few versions ago.
Plus it has a "full-screen mode", which allows you to focus on your writing, without any of the distractions in other writing programs. The program does cost US$13, but it is totally worth the price if one is a journaler. Visit their website to see for yourself.
They also make a version of the same software that works on Windows (WinJournal).