Back in 2011, I wrote about a project called Newer Super Mario Bros. Wii, in which a group of fans were making their own sequel/expansion for the Wii platformer.
Well, unlike most fan-created mods, these guys actually finished the job, and the game is now available for download.
Newer adds 128 new levels, has new bosses, visual tweaks and a whole host of other stuff.
It's free to download, of course, and can even be played on unmodded Wiis, but you'll need a copy of the original New Super Mario Bros. Wii for it to work.
Newer Super Mario Bros. Wii [Site]