A quick look at the tools and tables out there for D&D Encounters
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Encounter tables have been in D&D forever with some people loving them, others hating them and adventuring parties meeting grisly fates when something too challenging comes there way.
To have a look back at older tables check out the AD&D Dungeon Master's Guild from 1979.
The 5th Edition Dungeon Master's Guide has advice for creating random tables and I've written about Using Random Encounter Tables in D&D.
Combat and Non-Combat Encounters
If you are looking for combat encounters start with Donjon's elegant encounter generator or the more builder-styled Kassoon's combat generator.
Kobold+ Fight Club is my goto place for filtering monsters and it also features a random encounter feature.
I've got a 5e Encounter Generator with options for combat and non-combat and gives extras like location and activity. Dnd speak has a whole list of d100 encounter tables.
For a randomly generated monster try Darehart's Monstrous Codex.
For Ravenloft there is a Curse of Strahd encounter generator.
For NPCs there is a Guide to NPCs for DnD 5E
For Dungeon inhabitants, hazards and locations look at the Guide to D&D Dungeons.
Lastly I've created a slew of simple D&D encounter tables.
I'll get out of the way that lots of my pdfs at DM's Guild are Encounter related.
A Pay What You Want option is Random Encounters Expanded with 76 pages of statted encounters, with options for different environments.
For cities look for ideas in Waterdeep: City Encounters.
My most popular or different one is 200 Non-Combat Encounters