Agar.io
http://agar.io/#ffa
This is the first one I played and got me into them all. In it you simply steer with your mouse and gobble up any orbs smaller than you and avoid larger ones. The more you absorb the bigger you get. Theres also a W key to 'feed' little parts of yourself away from your main body. Its useful to either bait others into your trap or to pay off those that help you. Another key, the spacebar, splits you right in half over and over the more you push it. This helps in escaping with at least part of you intact or to jump out and grab a fleeing orb.
Slither
http://slither.io/
This is the one I play the most now. You're like a gummy snake and the more you eat the longer you get. Helpful for encircling enemies. In this one you need to avoid head on crashes into other snakes while you all eat little dots. Try to cut off others and make them run into you . When they do they turn into dots themselves. Player dots are bigger and glowy and you gain size faster by eating them.You can hold either your right or left mouse button to zoom around at turbo speed. Helps to cut others off like that.
Diep.io
http://diep.io/
This one is a tad different. In it you build up a gun platform that flys around. You shoot the stationary dots and target and earn exp and then you add points into different guns and addons for your 'gunboat' . Speed, health regen, bullet speed, power, and some more. This one can be completely different depending on how you build up your gunboat.
Deeeeep.io
This one is a bit more complicated. The .io games are known as simple yet addictive games but this one is like a life evolve one. Instead of getting bigger or adding guns to yourself you choose how to evolve biologically. Shark, crab, piranha. Do that until you're the top platypus in the pond
Its too in depth to really go into it but you get the hang of it fast....eat smaller biologics underwater and avoid being ate. Jump up to land and take things out there or hide from enemies that cant leave the water yet or cant fly.