Alternatives to classic games:
Elite -> Oolite
SMB -> Supertux Advance
MC -> Minetest (Dreambuilder/Nodecore)
WL3 -> Retux
Cannon Fodder -> OpenFodder
Arx Fatalis -> Libertatis
CMR2 -> Trigger Rally
SPang -> PangZero
HL -> OpenHL
Castlevania -> Edgar
Metroid -> Hexoshi
Diablo -> FlareRPG/FreedroidRPG
Zelda Lnk:Awk -> A decompilation it's there,
but an SDL port/remake it's badly needed.
DX/UT -> https://github.com/dpjudas/SurrealEngine (WIP)
Close to complete.

More engines and clones:
https://osgameclones.com/