Since this is a rabbit hole, we'll ease into it slowly by organizing it into major sections of the airframe and powerplant.
For a list of modifications for a specific aircraft type, start in the List of backcountry aircraft.
Avionics and instrumentation
Baggage and seating
Extended baggage, jumps seats, cargo options.
This will include engine upgrades and directory of popular STC's.
This will include tires, wheels, gear legs, suspension, tailwheel and nosewheel mods.
This will include propellers and directory of STC's.
This will include STOL wing cuff, flap, and wing tip/extension mods.
STOL kits usually include a riveted-on leading edge augmentation that changes the camber of the wing for better low speed flying characteristics.
VG's are small tabs that work like tiny fences to stabilize the air flow over the wing, and help prevent boundary layer detachment at high angles of attack.
Many aircraft are undergunned in the wing area department. Adding inches helps to produce more lift at slower speeds, given adequate power.