The bigger the aircraft, the more systems you'll find...

1) Propeller governors

The propeller governor is responsible for changing the pitch of the propeller, based off of pilot inputs and changing engine/propeller workloads. It works to maintain a constant engine RPM in flight.

2) Squat switches

These squat switches, or "weight-off-wheels" switches are responsible for activating or deactivating systems while on the ground. Squat switches may turn off lift detectors on the ground, turn off maintenance hobbs time, and prevent inadvertent gear retraction.

3) Unfeathering accumulators

Found in many multi-engine training aircraft, the unfeathering accumulator stores oil under pressure when you move a propeller lever into the feather detent position. When you move the propeller back out of detent, high pressure oil returns to the propeller hub, moving the propeller blades back toward low pitch/high RPM. This helps the propeller start windmilling again, and helps the engine to restart.

4) Advanced environmental controls

Systems like 35,000+ BTU Janitrol/C&D combustion heater burns fuel in a cylindrical tube, and keeps your cockpit toasty warm on cold winter days.

5) Hydraulic systems

Most aircraft use hydraulics to raise and lower landing gear. And in larger aircraft, the hydraulic system can be routed to manipulate control surfaces, like ailerons and flaps.

6) Synchrophasers

Synchrophasers match the RPM of both engines to reduce oscillation noise. Flying is a lot more enjoyable when you don't have to listen to that "wha wha wha" sound when the engines are out of sync.

7) Fuel cross-feed

A fuel imbalance can easily be solved by switching the fuel selector of the lowest tank to cross-feed. This allows an engine to draw fuel from the opposite fuel tank.

8) Deicing/anti-icing equipment

Whether it's deicing boots, weeping wings, or heated leading edges, there are all types of deicing equipment you can find on complex twin-engine aircraft.