Mario Builder is made by TheTingThing, over at YoYo Games Forums. As far as I know, this is Windows only. (If Mac users manage to play this on their Mac, let me know). You can download it here.
However, there is a major flaw. As far as I know with this, you cannot share your games over the internet. Someone must be with you to play your game.
When building your level/world maps, use CTRL to place tiles down rapidly, and the right click button on your mouse to delete things. When editting your world map, and you use more than a 1x1 grid, use W to move your map up, S to move your map down, A to move your map left, and D to move your map right.
Leave your thoughts here. Much appreciated!
Also one final note: if you can, give us screenshots of your maps, and maybe some videos if you can!