How to count the current number of users visiting the website?

[eluser]WanWizard[/eluser]
[quote author="n0xie" date="1277845397"]You could do a count on the session table if you use DB Sessions. It's not 100% accurate but it gives an indication...[/quote]
For that reason I added a 'last_update' field to the session table, which is updated at every page request. The last_activity field has nothing to do with what the name implies, it only gets updated when the session key is updated.

This also allows me to run queries like "who is currently logged on?".