John Romero, one of the creators of Doom, has released his unofficial fifth episode for the shooter, and you can play it now for free.
Sigil comprises nine new, labyrinthine levels for the original Doom. Each contains “more detail” than the levels in episodes 1-4, but Romero says they should feel like they “belong in the original game as if they were a true fifth episode”. They're playable in singleplayer, co-op and deathmatch.
The story directly follows on from episode 4: Earth is overrun with monsters and it's your job to teleport to Hell and confront the Satanic source before returning to Earth a hero. It's an excuse to blast waves and waves of demons, basically, which is fine with me.
The Megawad—the name given to particularly large Doom mods—is completely free if you don't mind a MIDI soundtrack, but you can also spend €6.66 (roughly $7.50/£6) on a version with a metal soundtrack from guitarist Buckethead.