DOOM 1: E[x]M[y] (X: 1-5, Y: 1-9) (9 is the secret level)
[Episodes: Knee-Deep in the Dead, The Shores of Hell, Inferno, Thy Flesh Consumed, SIGIL]
DOOM 2: MAPXX (XX: 01-32) (31/32 are the secret levels)
Master Levels: ML_MAPXX (XX: 01-21) (22 is the secret level)
nerve.WAD: NV_MAP0X (X: 0-9) (9 is the secret level)
The Plutonia Experiment: PL_MAPXX (XX: 01-32) (31/32 are the secret levels, 31 only has the super secret level exit)
TNT Evilution: TN_MAPXX (XX: 01-32) (31/32 are the secret levels)