Considered by many as "just another mod" for iD software's Doom, the FreeDOOM Project fulfills a much wider goal than it meets the eye at first. It offers a complete Doom IWAD 100% sprite-by-sprite compatible with the original game, that other modders can build on, just like the original Doom ones,...
It's still being actively worked on too.
I haven't played in a while, but every week or so I pop onto the git repo to see the new stuff. I especially love the music!