https://kobold.club/fight/ – Combat encounter generator. Generate combat encounters based on environment, alignment, difficulty, size. Allows you to select from different source materials, official and 3rd party.
Improved Initiative – Creature list, stat blocks, initiative tracking… all in one!
Creature Bestiary – Huge, searchable list of creatures on their stats.
How to Manage Combat Like a Motherf$&%ing Dolphin – Angry GM article on how to make combat flow like a boss… or dolphin.
The Monsters Know – Great blog that details tactics various monsters would employ during combat.
Improved Monster Stats – A table of creature stats by level to use on the fly. There is a lot of explanation behind the reasoning of the stats used. Adjust as needed.
D&D 5th Edition Monster Cards – Contains every monster available in the SRD and OGL, complete with form fillable template to make your own.
Combat Encounter Sheet – Character sheet like page for creature stat blocks and initiative.
3rd Party Monsters (Free)
Skip’s Guide to Monsters – A complete conversion of the 3e monster manual to 5e.
Forgotten Realms Conversion Guide – A conversion of the Forgotten Realms Monstrous Compendium to 5e by the maker of Skip’s. Creator admits it is not as polished.
Monster-A-Day – Subreddit dedicated to making new monsters. Works with Kobold Fight Club.
3rd Party Monsters (Paid)
Tome of Beasts – Supplement made by Kobold Press. Works with Kobold Fight Club.
Fifth Edition Foes – Supplement made by Necromancer Games. Works with Kobold Fight Club.
Monsters of the Orient – Supplement made by Marc Altfuldisch. Works with Kobold Fight Club.
Nerzugal’s Extended Bestiary – Supplement made by Nerzugal. Works with Kobold Fight Club.
Ultimate Bestiary: Revenge of the Horde – Supplement made by Nord Games. Works with Kobold Fight Club.
Goblinist Random Encounter Generator – Another combat encounter generator.
Blue Monsters – Monster pack affix ideas similar to how minions work in Diablo 3.