Alien Carnage (a.k.a. Halloween Harry) (some versions)
Arkanoid: Revenge of DOH (co-op paddle support in some ports)
Battletoads & Double Dragon
Bio Menace (unofficial co-op mod)
Bomberman (Dynablaster) (some versions include co-op vs AI)
Bubble Bobble
Bubble Ghost (2-player mod version)
Clowns & Balloons (unofficial DOS port with 2P mod)
Double Dragon
Final Fight (unofficial DOS port with 2P support)
Firepower
Golden Axe
Jazz Jackrabbit 2 (unofficial DOS builds with 2P support)
Liero (co-op via AI mods and shared-keyboard play)
Metal Mutant (dual-control co-op variant in later versions)
Popcorn (Breakout-style homebrew with 2P co-op)
Sokoban Co-op Mod (unofficial puzzle mod with 2 players)
Sopwith II (modded version allows 2-player AI base attack)
Stargunner (beta build only — not in final release)
Street Smart (arcade-style arena co-op)
The Incredible Machine (modded for two-player simultaneous input)
The Lost Vikings (modded for shared-screen 2P control)
Tyrian 2000
Xargon (homebrew co-op mod)