Hero Academy is the absolute pinnacle of “digital crack”. Summary: Free turn-based small-unit tactics game with classic sword-n-sorcery characters, power-ups, and a global pool of players. The new in-game tutorial video is very good, and there are a plethora of on-line strategy guides of if you need more help. There are low-priced in-app purchases of cosmetic and variety items, but if you are Lifehacker-cheap you don’t need to buy them to enjoy the game.
Even quicker summary: Totally and dangerously addictive. Get it.