Resources & Forums
ZDooM Forums: The best community of artists and developers
Wads in Progress: Compilation of most of the Wads in Progress of the net
Realm667: The best repository of the net, for DooM, Heretic and HeXen
DooM World: The main DooM Community on the net
ModDB: Most relevant HeXen Mods and Addons
Diseño DooM: Best DooM, Heretic & HeXen Community in Spanish
Sourceports
Skull Tag: One of the most elaborate source ports you can find
Doomsday Engine: Modern engine for playing classic 2.5d first person shooters, highly recommendable
Zandronum: Multiplayer oriented port that supports up to 64 players
ZDooM: The genuine, don´t need any presentation
Vavoom: One of the most advanced DooM, Heretic, HeXen and Strife source ports
HeXen Gles: Work in development, incomplete but very promising
WAD Editors
Slade: The most modern WAD editor, highly recommendable
DooM Builder: The best map editor for DooM, Heretic, HeXen & Strife
Extendable Wad Editor: Another classic and powerful editor but unfortunately stop to update
WhackEd: Windows version of DeHackEd, created by Dennis "Exl" Meuwissen
Eureka: WADs games map editor
DeHacked: The first one, where everything starts, in MS-DOS
Doomed: shareware sector based easy access level editor for Windows 3.1
BSP: The earliest and best nodebuilder for DooM and derivatives
Wikis
Heretic Wiki: Wiki about Heretic, HeXen and all the the Serpent Riders universe
ZDoom Wiki: Compendium of wisdom about the ZDoom engine, including information of sprites, decorate, scripts...
DooM Wiki en español: Best and more complete DooM wiki in Spanish
DooM Wiki: The most complete wiki site about DooM
Classic DooM: Great site dedicated to the Classic DooM of MS-DOS, without addons and with the original engine
Pixelart and Utilities
Piskel: One of the best free softwares dedicated to the pixelart. Highly recommended
Audacity: Great and simple software to the recording and edition of audios, including .WAV, .MP3 and .MID
Gimp: The free alternative to Photoshop, based in GNU, a really useful tool for the pixelart