Dungeons & Dragons official website
Dungeons & Dragons Adventures League - All about organized play (includes all rules documents)
Dungeon Masters Guild - Online supplements and resources
Basic Rules - FREE!
Dungeons & Dragons Beyond (D&D Beyond) - Online Tools for Players and Dungeon Masters
Pre-generated Character Sheets - New to D&D: Print off a first level pre-generated character and you're good to go!
Dungeons & Dragons Adventurers League Log (unofficial) - tool for keeping track of your AL characters online
Roll20 - Virtual Tabletop for playing games
Beyond20 - Browser extension to connect D&D Beyond with Roll 20