If you love those kind of games where you prepare food orders for customers in X amount of time, while upgrading the equipment, then Cooking Fever will be really fun for you. Haven't spent a cent and it's been good casual fun
Flappy Bird, Angry Birds, Candy Crush Saga, etcettera.
Jokes aside, you should get Clash of Clans!
I didn't really find it fun, so I quit after playing it for a couple of weeks..
But lots of people seem to enjoy it, and it's free!
Pocket summoner is my fav
also i have: Clash of clans, BTD Battles, Brave Frontier, Game of War, Underworld Empire, Heroic Legends, Minecraft PE, Zynga Poker, Crimsonland and every Kairosoft game
I just found an excellent free to play shooter that is 100% free. Its called critical mission. Its basically cod and counterstrike mashed into one game. Highly reccomend.