Quake has hundreds of user created levels - known variously as "mods", "total conversions" or simply "maps". To run new maps, place the bsp file into the "quake/id1/maps" subdirectory, and start Quake with the +map MAPNAME option.
Installing mods is simply a matter of creating a subdirectory DIR and extracting the contents of the mod zipfile/tarball into this directory. It is then loaded by using the -game DIR command line option and, sometimes, +map MAPNAME where MAPNAME is the mod's start map.
For both maps and mods, all files should be in lowercase.
Some of the more notable mods are:
This recent mod is a must have. It includes 15 single player levels, stunning bosses, and a couple of twisting spider infested caverns. Distrans has also contributed a professional quality soundtrack composed of moody and interesting electronic vibes. Link
Quoth began life as a sequel to the draw-dropping mod, Contract Revoked, but has recently gained much support from mappers, as is becoming the defacto extension of Quake I.
It's new features include numerous crazy enemies with strong AI, destructable environments, and other items including ladders and flashlights. Link
A huge atmospheric Quoth mod by Ijed. The latest and greatest. Link
Gorgeous medieval themed mod with two single player episodes, a melee style arena mode and nice documentation. Many thanks to Tronwyn, Fat Controller and their co-authors. Link
Epic. Ambitious. Otherwordly. Nehahra is the definitive Quake mod, supported in Linux by LordHavoc's Darkplaces, and QuDos' port of the NehQuake engine. It has some great models and maps, uses fog effects well, and has an (optional) two hour movie/demo which ties-in well with the original Quake "story-line" but is also in desperate need of an edit.
Nehahra also features improved enemy AI - the grunts strafe behind cover, and scrags in open space can be very hard to pin down. New gamers may wish to disable this feature by typing nomonsters 1 at the game console. Link
Dark and bloody, with Doom's paranoiac atmosphere. A classic. Zerstörer also has some good death match levels - Link
Neil wrote the definitive Half-Life mod "They Hunger". The quality and fun is in these maps too - alba01, alba02, sofsp1, sofsp2, starshp2, Link
SdS is a speed run through Mission Pack 1, Scourge of Armagon. Hilarious and amazing. Link
Scourge of Armagon by Ritual Entertainment (formerly known as Hipnotic Interactive).
After playing Armagon, you should also check out the Death's Dominion Level Pack.
Dissolution of Eternity by Rogue Entertainment.
Both official mission packs are generally acknowledged as better than the original game.
Very original Quake total conversion, with the greatest (virtual) machine gun I've ever unleashed. Worth paying for.
Commercial mod now freely available: Link
Xmen mod! Great models. Shame about the game-play. ...When good mods turn bad.
This mod has now been released free of charge, and can be found here.
One of the first commercial mods. It has well modelled and totally new monsters, but the game-play doesn't cut it.