Action Half-Life The best multiplayer mod for Half-Life, IMO.
Planet Half-Life A great Half-Life news and reviews site.
Valve Editing Resource Center Filled with tons of tutorials, tools, and resources. Check out their tools as soon as possible! ;)
Recommended Resource Sites:
RUST Has TONS of prefabs from various categories, and it's an okay mapping site.
The Wadfather An excellent site with textures and art for various Quake-engine games. If you're looking for a certain texture, chances are, it's here.
Counter-Map A Counter-Strike mapping site with some good urban texture packs. Worth a look for AHL mappers in need of new textures.