MyBB - Mods - Default Profile v2.0.1
This is good to have so that everyone will have an avatar, website, signature, etc. It makes your site look more full of content and you can also put links to your site in their signature or sell the space if you want.
MyBB - Mods - Adsense Revenue Sharing
This is something that will definitely encourage more users to post.
MyBB - Mods - Hide Links to Guests
Make guests sign up to see links. Increase your member count.
MyBB - Mods - RSS To Post v1.1.0
Make your board full of related content.
MyBB - Mods - Newsletter Plugin
Let your users know what's going on with your site.