3D Max just wouldn't be itself without plugins, and honestly a few good plugins and scripts save hundreds of hours of work sometimes, tens of hours on a regular basis. This is a list of what I use the most.
A custom city generator that I'm working on primarily based on analysis of work by tyson ibele
BGPlus, it's a building generator, now including a city generator with street plan etc
city engine, another type of city generator
cebas thinking particles
RealFlow (liquids both stationary and moving)
Sitni Sati package including Afterburn, Dreamscape, Enlight, FumeFX and Scatervl
The Foundry Nuke and Nuke X
Fracture Voronoi (for creating fractured objects for destructables)
Ghost Town (another city gen)
Ivy Generator (self explanitory, very niche)
GWIvy (a different ivy gen)
ADVPainter (used for painting directly in max on objects)
BuildingGen (yet another building generator)
Greeble (if you wanted to essentially make the star wars trench run in under a minute this is the script)
collision map generator
the fume fx source manager (strangely not included with my copy of fumefx)
PPainter (yes two P's, aka proxy painter, a kind of surface painter)
Whimsy Wunda Walker
I've probably got a few dozen other scripts and plugins installed but I know I've used all of these in the last month, repeats are for different perspective or methodology or missing parts. For example I can generate the same city several different ways to see what style suits best, or changes in altitude, road design etc.
While ZBrush is a stand alone program I consider it closer to a plugin. It substantially increases workflow and cuts time which is why I didn't include it in my previous post.
Let me know what plugins you use.