Apache's default directory index pages are ugly. Indices is a neat set of hacks that makes them pretty, and more usable.
Unlike other PHP-based solutions, Indices is based on Apache's built-in directory mechanism, so it's pretty easy to seamlessly integrate it into your entire website.
Indices uses a few helpful Apache mod_autoindex directives, coupled
with a bit of CSS and (optional) Javascript hackery.
The README talks about
installation and how everything works.
Also, I wrote a detailed blog entry that describes what's going on.
I developed Indices for my web server, which runs Apache 2.0.x and PHP 5.1.x. It'll probably require some tweaking to get it working with earlier versions of either, but it's doable.
If you're running Apache 1, you'll be able to use some of this code for inspiration, but you'll need to do a lot more hackery.
If you use and like this, feel free to buy a Wordclock Records CD. :)