Growtopia

3.65

Growtopia is a sandbox platformer MMO that brings together people from all over the world, which can either cooperate or fight. Read more about Growtopia  

You can think of Growtopia as a Minecraft-type game that takes place in just two dimensions that also incorporates trading and a massive communication hub that makes it easier to interact with other people.
In Growtopia you can build pretty much anything that you want, and that includes houses, dungeons, song, artwork, puzzles, and so on. You can also choose to cooperate with other people for a common goal, you play various mini games, and you need to protect your items as stealing is a major part of Growtopia.
Furthermore, players can also plant trees, trade items with other people, earn a single in-game currency entirely through gameplay, splice seeds to discover new items, get in PVP battles, and much more.