Useful sites

Sven Co-op Entity reference, contains a lot of information about 
point and brush entities, as well as useful technical mapping info.

Contains tutorials and Half-Life Entity Reference.

Tommy of Escondido's Mapping Help. Compile errors solution
described here makes this site of the most useful.

Some mapping tutorials for Half-life/ counter strike 1.6.
You can find some tricks here, and design theory.

The Legendary Website, with a lot of  tutorials, maps and files. Everything you need.

Unique tutorials about modding elements like creating custom textures, models e.t.c.- not only for GoldSrc.

An old site with some tutorials guides, with separate
Sven Co-op section!

Everyone knows Gamebanana! There is also a lot of textures
in cs-1.6 section- Here you go. A lot of wads/skies for you map!

A new editor I'm using for tutorials. If you are familiar with Valve Hammer Editor there should be no problems in working with Jackhammer. Try it out!

Currently Unavailable- Probably server is down forever...

Contains a lot of maps and files that can be also useful for
Sven Co-op mapping.