hammer help has some of the best info, If your a beginner start there.

https://twhl.info/wiki
http://www.mapcore.org
http://dod.gamebanana.com/maps
http://www.17buddies.net
http://www.snarkpit.net
http://web.archive.org/web/20071012233932/www.vlatitude.com/tutorials.php
http://articles.thewavelength.net
http://www.slackiller.com/tutorials/tutorials.htm
http://www.interlopers.net
http://www.dodplugins.net/forums/index.php
http://web.archive.org/web/20080527073453/http:/www.hlcu.com
http://zhlt.info/common-mapping-problems.html - (Compile Errors)
http://www.dodplugins.net/forums/cmps_index.php?page=verc - entity list
http://pastebin.com/f3kG6B6m dod texture list
http://pastebin.com/XmyfmuJX half-life.wad, gfx.wad, hlbasics.wad list
http://www.snarkpit.net/snarkpower/articles/game/cat/pn/tp/72/Day_of_Defeat_Mapping:_Part_1
http://game-artist.net/forums
http://forums.tigsource.com/index.php?board=16.0
http://forums.svencoop.com/showthread.php/38059-ARCHIVE-Custom-ZHLT-by-vluzacn
http://worldofleveldesign.com
http://nemesis.thewavelength.net
http://zhlt.info - (Useful for entity info further)

http://cs-mapper.com
Mundomapping.net (very friendly community)


http://jackhammer.hlfx.ru/en/main.html
http://sledge-editor.com sledge editor
http://quark.sourceforge.net/ quark
www.wantonhubris.com/toetag/Download.html ToeTag (Mac):
http://radiant.robotrenegade.com/


http://www.slackiller.com/hlprograms.htm
http://www.getpaint.net/index.html free paint program
https://developer.valvesoftware.com/wiki/3D_Modeling_Programs
http://khed.glsl.ru/ free 3d program, good for staic models
http://www.turbosquid.com/gmax (cut down version of 3ds max, if this works let me know)
http://www.milkshape3d.com
http://www.gimp.org free paint program
http://zhlt.info
http://twhl.info/forums.php?thread=18271 trimesh for hammer 3.5


http://www.hourences.com/product/the-hows-and-whys-of-level-design-2/


https://game-development.zeef.com/ellison.leao
https://developer.valvesoftware.com/wiki/Designing_a_Level
https://developer.valvesoftware.com/wiki/Multi-Player_Mapping_Tips
http://www.benb-design.net
http://www.pixelprospector.com/the-big-list-of-game-design/
http://level-design.org/
http://gangles.ca/2009/05/26/guiding-the-eye
https://www.johnsto.co.uk/design
http://www.optimization.interlopers.net (made for source but most of it is relevent)
http://www.worldofleveldesign.com/articles.php

http://web.archive.org/web/20080430160202/http:/home.att.net/~firebinder/hl/test1.htm

http://www.forbidden-places.net - (Good Reference Pics)
http://www.28dayslater.co.uk/forums/index.php - (Good Reference Pics)
http://leveldesigninspirationmachine.tumblr.com/
https://www.youtube.com/channel/UCEvEZ7egEEMOzcDS15jcGjA

Links submitted by Zero

http://gamer-lab.com/rus/goldsrc/Programmi
https://www.hlportal.de/?site=files
https://ficool2.github.io/HammerPlusPlus-Website/features.html
https://www.johnsto.co.uk/design/personal/#hl-etc
https://valvearchive.com/archive/Other%20Files/
https://developer.valvesoftware.com/wiki/Day_of_Defeat:_Source_Level_Creation