Today we have some tools I found to help drive people back to your site. 

Social Locker does just that, it locks content and the only way you can see it is to tweet, share, +, etc on social networks. This plugin costs $25

Show Down is a Hot/Not Rating kind of plugin used for content or pictures. It's a simple plugin that can have your audience helping your decide on your next logo, tag line, etc This WordPress plugin costs $23

The Photo Contest plugin does a similar thing to Show Down (it appears to have more anti-cheating built into it) It costs $41

(http://www.weeklywebtools.com/contestblitz) which is only $19