Monetized lockscreen (Cover + $$)
As Marc Andreesson astutely stated, "Ad tech has fueled a race to the bottom." This is especially true in mobile, where intrusive in-app ads frustrate users and generate almost no revenue. The clear answer is to advertise on the homescreen and lockscreen. Plenty have tried, but all have failed...because users won't give you that valuable real estate without a big-ass incentive. StartApp and Airpush tried by using various tricks, but were banned by Google. Locket and Slidejoy tried by giving users pennies per click, but the reward was too puny.
Enter AppKey, with a patent-pending incentive: give users free premium content in return for their permission to be shown geo/behavior-targeted ads on their homescreen and lockscreen. Users love it: free premium content, plus no more ads in the middle of their games. Developers love it: instead of pushing IAP, they simply unlock premium content to opt-in users and share in AppKey's ad revenue.
AppKey...mobile advertising done right.