A rumour started to fly around those attending E3 yesterday that a new Mario game was to make its debut on the 3DS during the show. Now, thanks to a "first look" during Nintendo's annual online livestream event, Nintendo Direct, we know what it is.
Mario Maker basically lets you create your own Mario levels and it's for Wii U, not 3DS. From the livestream though, it is unclear whether there will be a 3DS version too, which will allow migration of levels between both devices.
For now, the Wii U version lets you basically build your own Mario levels on the fly using the touchscreen of the Wii U GamePad.
Once complete, you can switch (it seems) between 8-bit original Mario graphics to New Super Mario Bros visuals. And we would suspect that there will be an active community of designers out there and a zone where you can download and play their wares.
While on display on the show floor at E3, the game/creation tool will not be available until 2015 - most likely the first half of the year.