GM Resources
This collection is made for GMs to get inspiration, guidance and tools they didn't know about beforehand.
If you want to add something just shoot me a mail at AimedZero@taw.net
So go ahead. Knock yourself out 
Useful websites
Cartographer's Guild: A great collection of cool, realistic maps for any occasion
Barrel, Crate, and Sack Generator
Reddit.com/r/DnDBehindTheScreen
Donjin: A set of random generators. Works for generic games, as well as PF and 5e
D&D 5E-Specific
Kobold Fight Club: Tool for making balanced encounters for DnD 5e
Orc Pub: Has character sheets, references to rules, monsters and much more.
Ambience Tools
RPG Ambience: A soundboard with sounds for any occation.
Ambience Mixer: Simple website with lots of sounds and ambience tracks.
Tabletop Audio: Has several music tracks, as well as a fe soundboards (beta). Integrated into Roll20
Blogs
https://theiddm.wordpress.com/
https://lootthebody.wordpress.com/
Useful YouTubers
Matthew Colville: Beginner series. How to be a good GM with lots of examples and resources.
DawnForgedCast: Beginner + Advanced: Lots of GM advice, as well as small discussions on classes, deities and more
Nerdarchy: dnd - Advanced: Many discussions about different aspects of the game.
TheOutsider68: Beginner + Advanced: Good introduction series on how to DM, as well as some advanced tips and tricks.
AFistfulOfDice: Good beginner GM series, based off of Pathfinder and DnD 5e
BeABetterGameMaster: A series of videos of how to be a better game master
Software
DM Minion: (PF&5e) Great tool for planning and playing. Has a treasure generator, combat manager, stat tracking of players, as well much, much more.
Combat Manager: (PF) Handles combat, initiative, hp of monsters and players, as well as displaying statblocks for most monsters in the bestiaries and Adventure Paths