Mac OS X 10.4 Tiger includes a lightweight database engine called SQLite, a free, embeddable, zero-configuration database system that is gaining popularity over the previous contender, Berkeley DB.
MacZealots has an article on installing Movable Type on a Tiger system using SQLite. I'm mostly linking it for its SQLite material, but MacZealots has published a MT-on-a-Mac how-to before, which uses MySQL, for a general audience. (You can also use MT with Berkeley DB quite easily, but SQLite looks like more fun to hack.)
SQLite is also a fair bit more stable. I'll never run MT on berkely again. I'm actually the first person who ever installed Movable Type on a computer not owned by Ben and Mena Trott, and so have pretty much the oldest existing install of MT ever. Before I finally converted to MySQL, my BDB database must have gotten corrupted two or three times, requiring some very nasty and scary database surgery and in some cases real data loss!
SQLlite is one badass little hunk of code.