If you seek tutorials about for a specific game, try visiting that game's website/forum/community for them, or ask around there.

A list of tutorials about QuArK and its functions:

Creating a map	View with Web-browser	This tutorial will guide you through the way the QuArK map-editor works, and how you should use it. Author: QuArK Development Team
Videos	View with Web-browser	Various video tutorials.
How to	View with Web-browser	The How to section on our forums.

A list of tutorials about mapping and modelling in general:

PlanetDoom Tutorials	View with Web-browser	"Need some help with that map you're making? Not sure how to make that Cacodemon appear out of no where and scare the crap out of everyone? well this is the place to learn all the in's and out's of mapping."
Frogbot Tutorials	View with Web-browser	Various Quake 1 tutorials. (Original site down; this is a mirror site.)
Inside3D - QuakeC Tutorials	View with Web-browser
quake2.com Tutorials	View with Web-browser	Some Quake 2 DLL tutorials.
Tiporium Quake Mapping Tutorials	View with Web-browser	Here you will find all kinds of tutorials, from making maps, to finding drivers, to diagnosing problems, and much more.
HL2World Knowledge Base - Source Tutorials	View with Web-browser	Here you will find a number of tutorials to help you create original content for Source Engine based games.
D-Day Dev Central Tutorials	View with Web-browser
How to make a Video Game	View with Web-browser	This tutorial shows you how to make your own video game for absolutely free.
The Archive Tutorials	View with Web-browser