Maybe a tutorial how to code a complete CMS system in OOP.

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Users online atm
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PM's
User control panel
Admin control panel
Usergroups
News system
Perhaps even using AJAX would be cool
Users-only pages
Users-only features
Memberslist
etc..

I'm sure you could think of more features..
But it would be awesome, very hard to write a tutorial though, cause it would be very VERY long, but nevertheless, very awesome :D