In terms of hybrid games that mesh together two or more different game genres into one title, Growtopia has to be one of the more stranger hybrid games when you initially think about it. The developers describe Growtopia as an experimental multiplayer sandbox platformer with crafting which just sounds really confusing at first.
Basically what Growtopia hopes to achieve is to provide a platform where players can create their own fun in the game. Starting up the game provides you with a choice of either picking a populated world or randomly generating your own. Once you're in your world you can begin harvesting resources by punching different objects which produces seeds. These seeds can be combined to make trees which you can then punch as well to begin harvesting better resources like doors, lava, jetpacks and so on.
Growtopia also has multiplayer functionality which allows players to not only chat amongst themselves but also to work together to build structures. The possibilities are pretty limitless as to what you can do in this game. The entire game is a universe of unlimited worlds all of which are persistent and connected to each other with doors you can build.
Right now the Android version is in beta, which you can snag off of Google Play right now if you want to, or you can wait till the official version of the game is released next week on January 11th, 2013. Either way it will cost you a whole lot of nothing to download Growtopia. Just keep in mind this is a beta so there will probably be a couple of bugs.