Browser-based multiplayer snakes!
Slither.io is a browser-based multiplayer game where you play as a snake (or maybe a worm? I'm not sure) trying to become the biggest of the bunch (while not becoming lunch). You accomplish this by eating pellets scattered around the map. Pellets are left behind as snakes move and when they die. Snakes die when their head bumps into another snake's body... and this is where things get interesting.
Timing and movement are important, as you can trap or force other players to run into you. While being larger provides a few more strategies (such as coiling around smaller snakes), you can't escape as easily as a smaller snake and have the downside of not being able to see your entire body. Timed correctly, even a tiny snake can dart in front of a giant and take it down. It's not uncommon to see chain reactions where a smaller snake takes out a behemoth and everyone scrambles into each other trying to gobble up the spoils.
Much like the game's concepts, controls are easy and straightforward. You point your cursor in the direction you want to go, and you can move fast by holding down a mouse button (which will shorten your snake as you use it). Controls are a bit different for touchscreen devices, but are still quite simple to use. An important note that new players might not realize is that you're allowed to cross over your own body. Be careful around snakes of the same color though, you might mistake their tail for yours!
This game is fairly simple to understand and easy to play, but has several interesting tactics you learn about as you go. It's not a mechanically deep game by any means, but I was pleasantly surprised by some of the subtler strategies available here.
Slither.io is often compared to the massive multiplayer hit Agar.io and the classic Snake game. While it combines elements from both of those titles, it adds a new twist or two (such as coiling over yourself) that makes for a unique and improved experience.