DM Resources

General

 * D&D Beyond - This digital tool for 5th edition can be used for looking up anything in the public SRD.
 * Not super useful right now but they are planning to add full character and campaign support to it eventually.


 * Reddit's r/DMAcademy and r/DnDBehindtheScreen - A resource where DMs talk to each other about DMing issues. Many topics already discussed, or feel free to ask your own!
 * Roll20 Help Wiki - Roll20.net is a powerful platform for D&D 5e and learning how to use it to your advantage can take a lot of the hassle out of GM'ing.
 * 5e OGL Compendium
 * 5e OGL Character Sheets
 * Roll Macros
 * Dynamic Lighting


 * Massive DM Toolkit

Encounters

 * Kobold Fight Club - The best encounter builder and XP calculator for 5th edition hands down.
 * The first rule of Kobold Fight Club is "Yip, Yip!"

Spells

 * dnd-spells.com - This site definitely breaks many copyright laws, but until D&D Beyond gets access to the full release content for 5th edition, we'll make due with what we can.

Random Stuff

 * Donjon.com - The quintessential place to go for all things random. You can even generate entire random dungeons.
 * Fantasy Name Generator - self-explanatory. More options than DonJon and you can choose by genre, setting, etc.
 * Massive and Growing List of Random Resources from /r/DnDBehindtheScreen - random tables for almost everything.

Items

 * All Magic Items by Rarity - A useful list for populating your dungeons with treasure. Make sure you check the Dungeon Master's Guide on how to balance the amount of treasure.

Dungeons and Battlemaps

 * Cartographer's Guild - The largest community online for aspiring dungeon artists. Go as deep as you please into this rabbit hole and make some awesome maps!
 * Reddit's r/Battlemaps - They have dungeons, as well as general battlemaps (fields, etc) - and they take requests!
 * Roll20's Marketplace - Most sets are paid, but they do have free things on here from time to time. Even the paid sets are usually reasonable (in the $5 range)
 * 15GB of Maps and Assets - Someone put together everything they've used over the years. Check the comments, more people added their resources as well.

Art

 * Tokens! - So many great character tokens here for monsters and players alike!
 * Fantasy Art - A whole bunch of character art to get your imagination rolling.

Homebrew

 * Homebrewery - Homebrew content is generally not allowed in the Whitereach setting, however, if you want to make something you're designing as a DM super pretty, check this out.

Roleplay

 * Accents - An amazing and free database for English accents from all over the world, figure out your character's own unique oral flair with this resource.

Read Other DMs!

 * Sly Flourish (the Lazy DM)
 * The Angry GM

Watch/Listen to Other DMs!
If you're new to the DMing game and want some inspiration, or you're just looking for some great D&D content, check these out:
 * Critical Hit (D&D 4e with forays into other systems)
 * DM: Rodrigo Lopez
 * The Adventure Zone (5e)
 * DM: Griffin McElroy
 * Nerd Poker (5e)
 * DM: Dan Telfer
 * Acquisitions, Inc. (5e)
 * DM: Chris Perkins
 * Critical Role
 * DM: Matt Mercer
 * Matt Colville's "Running the Game" series
 * Rollplay: Court of Swords
 * DM: Adam Koebel
 * He also does a DM help show called Office Hours
 * They also do a multitude of other games and Adam really shines in his character work.