MacJournal is one of my favorite applications. As you would expect, it is a journal application. You can use it as a diary, as a place for class notes, just about anything.
MacJournal comes in two flavors, free and not free. Version 2.6 is still free, but there is 3.x version that is marketed by Mariner Software. That one will cost you money. The paid version has more options, but the free version does all that I need. And that is, a place to keep a journal.
With MacJournal you can post to your blog from your entries and have multiple journals to write in.
Another rewarding way to use it is to record your life history one day at a time.