GM Tips : Geek & Sundry's Youtube show for DM/GMs, featuring fantastic advice from big RP names like Matt Mercer, Satine Phoenix, and more.
Matthew Colville : A Youtube channel created by a very well-respected, long-time DnD player about running Tabletop games. Invaluable advice for DM/GMs can always be found here.
Obsidian Portal : A website which allows you to create wikis for your campaigns/storylines.
Creativichee : A catalogue of various generators, including monster, personality, alien society generators and more.
Donjon : A catalogue of various generators, including world, name, item, encounter generators and more.
Fantasy Name Generators : A large catalogue with name generators for generic fantasy, specific settings, real life, and more.
Minotaur's Character Sheets : Several generators for character sheets, kingdoms, campaign planning, and more. Intended to be used for DnD and Pathfinder, but the kingdom and campaign planning generators could be useful for various games.
Plot Generator : A generator for storylines and plotlines.
Ritual Details Generator : A tool for creating magical rituals.
Minotaur's Character Sheets : While this site is mainly a generator for character sheets and such, it has several useful printouts of blank grids on which you can build battlemaps.
Old Maps Online : A catalogue of real life, historical maps.
Dungeon Painter Studio : A program that allows you to create both overland and detailed indoor maps.
Roomfull Express : A room planner that can be utilized to create indoor maps for your games.
RollforFantasy Map Creator : A simple and easy-to-use map creator for world maps.
Settings and Systems
13th Age SRD : A rules reference site for 13th Age, an alternative d20 game created by some of the developers of DnD 3e and 4e.
d20 Hero SRD : A rules reference site for the superhero tabletop game, Mutants & Masterminds.
d20 Modern Wiki : Find the d20 Modern SRD and homebrew content on the main page of this DnD Wiki.
Dungeon World SRD : A rules reference site for the DnD-like Kickstarter success, Dungeon World.
Sword and Wizardry SRD : A rules reference site for the nostalgic game copied from the earliest Dungeons and Dragons content.
Dungeons and Dragons
3.5 Edition SRD : A rules reference site for DnD 3.5e which also includes homebrew content.
5th Edition SRD : A rules reference site for DnD 5e which also includes homebrew content.
Blog of Holding : A creative DnD blog with tons of good tips, ideas, educated opinions, and inspiration for DMs.
DnD Wiki : A wiki for 2e and up.
Dungeon Master's Guild : An online store with DnD content created by DMs like yourself. You can sell your own original DnD adventures, monsters, player classes, etc on the Dungeon Master's Guild as well.
The (Not Really) Complete Tome of Spells : A massive list of spells converted from 2e to 5e.
The Pike Horde : A Reddit thread about an alternate view of Orcish warfare.
Elven Phrases : A well-organized list of useful Elven phrases.
Pathfinder Reference Document : A rules reference site for Pathfinder.
Pathfinder SRD : A rules reference site for Pathfinder which also includes homebrew content.
Coruscant Translator : A fan-made translator for various Star Wars languages.
Epic Worldbuilding : A website focused on creating settings for RPGs and creative writing which includes many articles and resources of its own.
Springhole.net : A list of helpful articles regarding writing, roleplay, and worldbuilding.
Adventure Scents : An online store where you can buy scents such as Ancient Library, Dank Dungeon, or Steam Train to enhance the atmosphere of your tabletop games.