I enabled PHP and installed MySQL on my Mac at home. Then I set up Drupal. It was really pretty darn easy. Even if you're afraid of the command line, it's not that hard when you follow these step-by-step instructions at MacZealots.
My only advice for anyone wanting to try this: do follow the directions and install version 4.0, not the latest version. I installed the later version and ran into the same problem described in the comments at the bottom of the article.
So setting it up was easy enough. It worked, looked great, etc. Only now I want to make it available to the outside world. Getting a dynamic DNS hostname was easy enough, and forwarding port 80 to my Mac's IP address was a piece of cake (I did have to assign my Mac a static private IP address, though). So now my Mac at home is a Web server.
Problem is, when I visit the drupal site, somehow it tries to resolve the URL to my local hostname. Which makes sense since I originally configured it with base URL http://localhost/drupal. But after changing that line in settings.php and restarting Apache, it still doesn't work. See what I mean?