How to display if a user is online?

[eluser]Tyler Diaz[/eluser]
I'm currently experimenting with Code igniter (and PHP) and I am trying to build a simple forum system.

I was wondering how do developers do to show if a user is online or offline on topics and profile. I am using http://dialect.ca/code/ci-simple-login-secure/ - Simple login secure to do the basic login job for me along with using the Code igniter custom session database.

So any leads, tips, advice, snippets, help on how this technique is achieved?