Neverwinter Nights and Neverwinter Nights 2 modders have ported a large number of pen-and-paper D&D adventures using the toolsets.This, among other things, makes the game much more stable, allows the addition of quality of life changes and bug fixes, and, most importantly, adds modding support.
Just as openmw, it requires the originals' game resources (though Daggerfall is available for free legal download).
Wario Land SNES a full remake of the first Wario Land game and tons and tons of remakes of Super Mario Bros. Some notable ones include Castlevania Mario Edition and Castlevania Dracula X Remake, TWO remakes of Castlevania: Dracula X using the SMW Engine. There are many, many remakes of older games based on SMW.Although less active now than in the past, in 2010 they released remakes of Knight Lore and the isometric Batman from Ocean (the original was made by the same authors of Head Over Heels). RetroSpec are a group who spent a lot of time recreating old ZX Spectrum and C64 games (their Head Over Heels and Wizball are awesome).Meaning there are probably more remakes of this game alone than any other (not that many of them get popular). Some colleges have courses where students must create a Pong game early in the semester.The former in particular is probably the single most remade game in the history of video games, and probably also sees the greatest number of cease-and-desist orders. The umpteen-million free remakes of games such as Tetris, Pac-Man and Space Invaders.
Cracked gives us a rather handy list with 5 Improved Versions of Classic Games That Fans Made For Free.Those probably fit better under Game Mod. Note: This doesn't count games where the creator has been granted official access to the source code.