I think it can be most useful, here you're can find most interesting things - skins, models, mods and other for your HL game.
Awesome website about hl game series. Hundreds of models, textures and sounds right here http://hl-lab.ru/
Popular gamesite, here you're can find simple tutorials, models, textures, sounds, sprites for your hl game and for dozens games like css, rtcw, cod http://www.gamebanana.com/
http://detler.ucoz.ru/ - it's better choise, if you want to create something it Valve Hammer Editor. Better tutorials and experienced modmakers here.

And now i want to give for you some link, where you can download most interesting hl mods in one time:
http://megatorrents.org/forum/viewtopic.php?t=274820
http://megatorrents.org/forum/viewtopic.php?t=361825

Update new links soon.