TroikaTronix Plug-ins
Mac OS

/Library/Application Support/TroikaTronix -> Isadora Plugins, Isadora Controls, GLSL Plugins


Windows

C:\Program Files (x86)\Common Files\TroikaTronix -> Isadora Plugins, Isadora Controls, GLSL Plugins


FreeFrame Plug-ins 
Mac OS

/Library/Application Support/FreeFrame


Windows

C:\Program Files (x86)\Common Files\FreeFrame


Quartz Composer Plugins 

Mac OS Only
/System/Library/Compositions
/Library/Compositions
~/Library/Compositions


User Actors 

This file path is user-defined and kept in Isadora's global preferences. It is initially not defined.

You can set your file path for User Actors within Isadora, under Actors -> Set Global User Actors Folder...