New from PHPit.net today, there's this tutorial on the creation of a "who's online" script with PHP.

In this tutorial I will show you how to create your own "Who's Online" script, which is often found on message boards and other community scripts. We will first create a really simple script, and after that add a few more features like the location of each visitor. But first, let me explain how our "Who's Online" script is going to work.

The script stores an IP and a timestamp in the database, and will keep note of their last page request. After a bit, when they are no longer making requests, it will remove them from the list. The little trick behind it is a "web bug" - a small script that actually runs as a small PHP-created image...