I cannot stop playing Splatoon. However, if you have a Wii U and are able to play Splatoon, odds are you already have it anyway. For something that most people can access but relatively few know about, I'd point to Town of Salem. It's free to play in a browser and runs $4.99 or less on Steam, but there's no real incentive to buy it unless you feel like supporting the devs. It's similar to the party game Mafia/Werewolf/whatever other names it goes by, in that there is a majority of town members trying to survive by finding and lynching the minority of mafia members before they're all killed. There are a lot of individual roles that make it more complicated, but that's the basic idea. It's all about deductive reasoning and being a good liar. And being able to tolerate the occasional abuse of Lenny faces.
Played Town of Salem over a year ago for about a week straight. Some roles were incredibly fun to play. Making a scheme and dropping subtle hints to mold other players' perception of you, frame an innocent townsperson, or get someone to use their ability at the perfect time for your interests. Really great experience. That "oh shit" feeling when ten living townspeople turns into three, after a single night. Hanging someone on a tiny whim and, bam, he turned out to be the serial killer. Piles of irony in that game.
Love Splatoon. The neat thing about Splatoon is because it is a team game the stuff I learned from TF2 and other team games carried over into this game. The downside is it only has half of the content on the disc is available right now and the rest are being pushed as "weekly updates". The 3 minute matches and 4 hour map rotation makes it difficult for me to stay and play for any period of time. Town of Salem is pretty neat if you can make it past the first round. It's one of those time killer games where you can just keep playing and playing. Agar.io is another one of those time killers that is free. Warning May lose several hours playing.