--General--
Official D&D website: http://dnd.wizards.com/
Official organized play website: http://www.dndadventurersleague.org/ 
Errata and dev rulings: http://www.sageadvice.eu/
Discussion: http://4chan.org/tg/ -- Search for 5e general threads! (/5eg/)

--Devs on Twitter--
Mearls (designer, "rule of cool" dev): https://twitter.com/mikemearls/
Crawford (official rules dev, his word on RAI is considered final by WotC): https://twitter.com/JeremyECrawford/
Perkins (the story guy): https://twitter.com/ChrisPerkinsDnD/

--Spells--
http://hardcodex.ru
http://www.dnd-spells.com
http://www.dndmagic.com/index.php
http://ephe.github.io/grimoire/

--Encounter Builder--
http://kobold.club/fight/#/encounter-builder

--DMG Tables and Other Tools--
http://1-dot-encounter-planner.appspot.com/

--Homebrew--
https://www.reddit.com/r/UnearthedArcana/
https://www.dandwiki.com/wiki/5e_Homebrew
http://www.enworld.org/forum/content.php?1883-Fan-Content-Database