Welcome to the MODBOX. This section of my website has mods for a few games that I've enjoyed growing up.
A lot of the mods I haven't even tried out, but this is for reference only. I do have some mario mods I myself made which are under the mario section (obviously).
The Super Mario page also has all of the editors possible to use as well as Super Mario Bros X, a free game for PC that also lets you create full on levels and games.
The Commander Keen page doesn't quite have the level editors linked yet, but has all the mods as well as a link to Commander Keen 64, a brand new fanmade game made to look like Nintendo 64.
Next, some Sonic The Hedgehog mod links, as well as Wolfenstein 3-D mods and editors, Ravengames (Heretic, Hexen, Hexen II, Heretic II) and finally (for now) Quake!
Soon I am looking to include Duke Nukem 3D, Half-Life 1+2, and possibly even The Legend of Zelda. Hell, there may even be a bunch more before I consider this area completed.