Brian Deshong has this new post on his blog today with an alternative method for doing the old standby "who's online" for your website.
Whenever you Google around for things like "Who's Online php", you'll find that a lot of the solutions are centered around using a database. However, is this really necessary? [...] One goal here: get Who's Online functionality off of the database. We'll explore a possible solution with Memcached that I've personally implemented, and thus far, it’s been working great.
His alternative, using the memcache functionality that can be enabled in PHP, stores a "user ID" (either a real user ID or just some unique identifier) and their last activity time. This information is stored in a local file that can be queued each time the page it loaded. The other piece is a backend script that goes through this same file and clears out the old values. Depending on the size of the file/traffic of the site, this process could even be integrated into the same script that writes it.