I found this app on the AdJoe offerwall in BitcoinAliens' Free Litecoin app (referral link). Cash Alarm works in a very similar fashion to AdJoe. One does not need to actually play a game downloaded through Cash Alarm to receive credit, but simply have the game running. The notable difference between AdJoe and Cash Alarm is that, in this app, the amount of time between credits does not increase. For every five minutes you have a Cash Alarm game open, you receive coins (100 coins ~ $0.01 USD).
The amount of coins you receive every five minutes starts at 100 and increases by 10 for each consecutive day you run the same game. Coins can be cashed out to PayPal or used to purchase gift cards for Amazon, PlayStation Store, BJ's, Starbucks, Nike Digital, T.G.I Friday's, Walmart, XBox Digital or Google Play. Interestingly, the more coins you save before cashing out, the bigger the discount you get. A cashout of $0.50USD to PayPal costs 4999 coins, but a cashout of $20 costs 182,667 coins ($18.27 if you cashed out $0.50 at a time). Also, according to Cash Alarm, there is no limit to the number of coins that can be collected for any one game, meaning that you won't have to worry about capping and having to switch games, eventually exhausting all available options.
PayPal cashouts apparently take 2-7 days to process on average, but I personally don't mind waiting a while for free money as long as I actually get it. I have already requested my first $0.50 cashout and will come back with an update post if and when I receive it. Until then, if you'd like to get started earning coins with Cash Alarm, you can download it here (referral link). Finally, Cash Alarm does not contain ads, though I'm sure most if not all of the games do. The best method for earning with this app is, in my opinion, to open it and run the game of your choice while you sleep.