MyBB - Mods - Custom Pages
You can make an affiliates page to list websites you want to do link exchanges with, instead of taking up space elsewhere[on the index page, etc.].
MyBB - Mods - IPLog Plugin
Keep track of your members easier this way. Make sure they're sticking to one account and not making other accounts to spam users or other things. Also, keep track of those banned and or troublesome members!
MyBB - Mods - Posts Required To View This Forum
Get more visitors to sign up this way.
MyBB - Mods - Side Boxes! 2.0
This one opens up a few spaces for you to run ads and get more revenue.
MyBB - Mods - Contact+ 3.0
A prettier and much easier way for your users to send you messages and for you to receive them.