Before we continue our discussion I think it would be useful to step back and consider exactly what a world engine is and what it is not.
For our purposes, there are three types of rules:
First up are the most common type, Direct Interaction. These rules dictate the success or failure of the players to achieve some action or to have something occur to them. Combat, spells, monsters and traps all fall under this distinction.
The second type of rule is a World Building system. Dungeon stocking charts, subsector maps, random monster generators and the like exist to help the DM populate their world. While World Building systems are an interesting and underdeveloped field, they exist to tell the DM what the world is like when the game starts.
Finally, the third type, World Engines, tell us how the world is changing around our PCs. These rules may affect the PCs in some ways (and usually do!), but exist primarily to demonstrate that the NPCs and game environment have a reality beyond the players.
World engines can be divided into two major types: environmental changes and NPC actions. Environmental engines allow for the mapping of complex systems such as weather and public opinion. NPC actions are often smaller and more personal in scale, such as wandering monster charts and how close the fuzz is to breaking down your door.
Below is a list of the world engines developed for various tabletop RPGs. This is obviously incomplete, and I'd love to hear more suggestions in the comments. In the near future I'll be analyzing a number of these to see what we can learn from them.
Basic D&D (Mentzer): Wandering Monsters / Monster Reactions
Marvel Super Heroes RPG (TSR) / MX1: Nightmares of Future Past: Search Flow Chart
OD&D / Ready Ref Sheets: Proclamations, Boons and Duties, Crime Trial & Punishment, NPC Cutups
Top Secret: Complications
Top Secret S.I. / Commando: Friction
Gangbusters: Public Opinion / Heat
AD&D / Oriental Adventures: Events Charts
AD&D / Dune Trader: Market Forces
Warhammer Fantasy Roleplay / Middenheim: City of Chaos: The Long Arm of the Law
GURPS: Contacts, Allies, Patrons, Enemies, and Dependents
Grognardia's Dungeon Restocking and Rival Adventuring Parties: here
Cole (of Abraxus fame)'s Evolving Villain Plans: here