My personal favorites of Old School.
Doom
Quake
Duke Nukem 3D (Atomic Edition)
Shadow Warrior
Worms 2/Armageddon
Monkey Island 2: LeChuck's Revenge
Might and Magic III, IV, V, VI
Albion
Starsiege: Tribes
Quest for Glory IV
Spellcasting 101, 201, 301
The Settlers II
Thief: The Dark...