Back in the Beforetime, Quake amazed people a great deal due to one little detail: It was amazingly moddable due to the use of QuakeC - you could make something completely amazing and complicated, and not just mess with graphics or levels!
(Should make a better list of Most Amazing QuakeC Mods, but this is a start...)
- Threewave CTF
- Team Fortress - my all-time favorite net FPS mod. Reborn as TFC for Half-Life (not as amazing) and as Quake 3 Fortress for Q3A (would say it's better than the original if that wouldn't be blasphemy =)
- Other popular mods such as Rocket Arena (a "duel" mod with less absurd rules) and Painkeep, neither of which I have tried...
...and then some mods that really screwed up the brains of people who saw them. How the hell can you make a first-person shooter do that?
- Quake Rally and Checkered Flag: GoldCup - Racing with Quake engine?... (QR has also been "reinvented" for Q3A)
- Quess / Quake Chess - Bringing a new meaning to "battle chess"...
- Target Quake - Quake platformer? Now we're going to the asylum...
- Several skateboarding mods, none as cool as Street Surfer though... =)
- Slide - hoverboard mod
Since the Quake source code was released by iD Software later under GPL license, we now have several odd mods and game projects for Quake that also mess with the source code itself:
- ttyquake - AAlib version of Quake. This one existed even before the source release, though (dynamic loading hack for svgalib)...
- wmquake - Your very own little quake window
- NPRQuake - New meaning to "graphical violence"
- ARQuake - Paintball sounds awfully boring after this
- Tenebrae Quake - You want cool shadows for scariness? Why wait for Doom III?
- MozQuake - at the moment very theoretical project for running Quake in... Mozilla window?
And then the entirely hypothetical:
Quake has also inspired art, in form of Quaiku. There are also numerous Quake movies (mostly in form of recorded game demos); One of the most amazing ones is Quake Done Quick with a Vengeance.
Thanks for additions to fondue