Dungeon Master Resources
- Player cheat sheet - printable reference with rules that are commonly looked up (status conditions, calculations, etc). Great for keeping book-opening to a minimum.
- Player reference card - Info summary about all players present, useful for the DM when tracking statuses, hit points, etc.
- 5e Dungeon Master Screen - Web page version of a DM Screen
- The Grimoire - Searchable spell list
Encounter Building and Tracking
- Kobold Fight Club - Encounter builder
- Searchable online bestiary
- The Monsters Know What They're Doing - Tactics for monsters and NPCs.
- Improved Initiative: Web-Browser based Combat tracker and compendium (monsters/spells/custom encounters/etc). Allows import of all the DnDAppFiles resources as well so you have access to all published material. Persistent and exportable (just to be safe).
- Hero Lab: Not free, but a very good character and NPC builder. (PC/Mac)
- donjon's Random Dungeon Generator (also populates with encounters)
- Anamap (Flash)
- Dave's Mapper
- Dungeon Painter Online (Flash)
- Tiamat the Tile Mapper
- The Angry GM: How to Manage Combat Like A Dolphin: A solid article on managing combat and making it interesting
- GM Tips: Geek and Sundry's ongoing Game Master tips videos hosted by Matt Mercer and more recently, Satine Phoenix.
- Running the Game: Matthew Colville's step by step guide to being the dm. Very thorough (several videos in playlist).
- WebDM: Weekly show about DM'ing and D&D in general.
- How to be a Great Game Master: Comprehensive tips on how to be a better DM and a better player