Here there are some links to the producers of stand-alone ECUs, that can be used instead of the original one, with more or less modifications of the existing system.
Australia
Motec http://www.motec.com/
Autronic http://www.autronic.com/
Adaptronic http://www.adaptronic.com.au/ (this is my choice!)
EMS http://www.fuel-injection.com/
Haltech http://www.haltech.com.au/
Microtech http://www.microtechefi.com/
Wolf http://www.wolfems.com.au/
United Kingdom
Pectel http://www.pectel.co.uk/
Emerald http://www.emeraldm3d.com/
Omex http://www.omextechnology.co.uk/
RTD http://www.racetechdev.co.uk/
MBE http://www.mbesystems.com/default.html
Switzerland
Ontronic http://www.ontronic.com/
Italy
EFI http://www.efitechnology.it/main_it.html
TDD http://www.walbro-tdd.com/
United States of America
EFI Technology http://www.efitechnology.com/
Electromotive http://www.electromotive-inc.com/
FAST - XFI http://www.fuelairspark.com/
and for the DIY enthusiasts, the great Megasquirt http://www.megasquirt.info/