Dungeons and Dragons Dungeon Master's Guide (Core Rulebook, d&d Roleplaying Game) by 
Dungeons and Dragons Player's Handbook (Core Rulebook, d&d Roleplaying Game) by 
The official site for all things Dungeons & Dragons, a completely searchable database of the core rule set, customizable digital character sheets, and even tools to create content for your own campaign.
Is a "virtual table top" site allowing you to play Dungeons & Dragons (as well as a bunch of other table top RPGs) with your friends over the internet.
A great site DMs full of randomizers that allow you generate content for your players on the fly. My favorite is the dungeon randomizer.
A no-frills database of the core rules for 5th edition, handy for use on a mobile device.
Downloadable pdfs of character sheets available for free from Wizards of the Coast.
A massive collection of adventures and other content, many of which are free or pay-what-you-want.
A Dungeons & Dragons subreddit full of useful advice.
The wildly popular live play web series.
Another wildly popular live play podcast.
The single most useful YouTube channel for aspiring DMs. Full of useful information in a clear and easy to follow format.