The EnerGI Maestro is a custom circuit board that takes the 4 General Illumination (GI) strings that are normally controlled by a single relay (you all know the clicking in the backbox right?) and controls them individually via a microcontroller with solid state electronics and high speed PWM. This is important if you are running LEDs in your GI sockets.
What does this mean? Well for one, you have a lot of control over the brightness and ramp rate:

Each GI String is individually programmable
Set the maximum brightness for each GI string of LEDs
Set the minimum brightness for each GI string of LEDs (you can elect for dim instead of fully off)
Set the rise speed and fall speed for each GI string of LEDs (full range, from fast to a very slow, several seconds of rise/fall)
Push-button control for easy programming on the fly
Plug and play
No more seizure multiball! Slow those LEDs down. Kep them from extinguishing all the way to provide a more pleasant flash that saves your eyes and improves your gameplay. Make your LEDs act just like incandescent bulbs. Stop that backbox from flashing, and bring its brightness down. You can do any or all of these things and more!

Visit this Pinside Thread to read all the details, and see some boards installed in users' machines.

Still not sure what exactly it does? Take a look at the Video page for some before and after action shots.

COMPATIBILITY LIST

TESTED / CONFIRMED TO BE COMPATIBLE
Back to the Future
Hook
Lethal Weapon 3
Jurassic Park
Last Action Hero
Tales from the Crypt
Baywatch
Guns N Roses
Star Wars
The Who's Tommy
Batman Forever
Checkpoint
WWF Royal Rumble
Robocop
Torpedo Alley

EXPECTED TO BE COMPATIBLE
Laser War
Secret Service
Time Machine
Playboy 35th Anniversary
Phantom of the Opera
Simpsons
TMNT
Batman
Star Trek 25th
Adventures of Rocky & Bullwinkle
Maverick
Mary Shelley's Frankenstein

POSSIBLY COMPATIBLE
High Speed
Grand Lizard
Road Kings
Pinbot
F-14 Tomcat
Fire