You want Quake and you want it now? Then downloading the full version would seem like a good solution.

• You can download it directly from id Software. No longer available.
• You can buy a digital copy of Quake and its mission packs at GOG.
• You can add it to your Steam account. This does not include the The Quake Soundtrack.
• You can download it from abandonware sites or torrents but you would be a sorry excuse of a fan to do so. Quake might be old but it is still being actively sold by its creators and thus it is definitely not fitting the definition of abandonware…

Also do not fall for the logical assumption that these options include bugfixes or proper support for modern operating systems. But do not worry about that, check out the Recommended Quake Engines.

If you just want to check this game as a first timer, consider downloading the shareware instead. It is free and includes almost one quarter of the game (but again, no soundtrack). You will not be able to play custom maps or mods with the Shareware, it will tell you "You must have the registered version to use modified games."