MyBB Community Forums

It would probably be too processor-intensive to work out precisely how many new threads & posts there are in each category, but you could have an indicator image like the forum indicators to say whether there are new posts or not quite easily. (In this case, the feature would not be unique to MyBB - SMF has it)

It would add about 1 query per collapsed forum.

Unless it was a GROUP BY query then, but once again that becomes a bit more server intense. I would like this as an option to show for all forums too and it is something I will be looking into as an option for future version

Yay! I made a good suggestion!

I didn't know that it has been in other message boards though..

Even if there was just an indicator icon, that would be cool too.