AdDuplex, a cross-promotion network for WP apps, has a referral program in which new users of adDuplex receive 5% more impressions of their own ads shown on other apps when they register through a referral code, for 6 months. So here is mine. (I'll get some free impressions too!)

Adduplex referral link for new users:

Get +5% free impressions for 6 months!