"Kyle's friend w/ 86"
It can be either a fuel piggyback system or a full standalone, depending on how you build it and implement it. It can control anywhere from a 1 cyl lawnmower to a 16 cyl engine, fuel only, or fuel and spark, or if you really want to, spark only. There are pre-built systems offered for sale for anywhere from $300 to $600. However, if you have patience and good soldering skills you can have a full standalone that can handle a bunch of extas (nitrous control, launch control, 3 sequential shift lights, electric fan control, water injection, etc...) for under $200. My kit is a Megasquirt 1 Version 3.0 circuit board.
Kyle brought up a good point, the ECU is pretty awesome and can do alot, however if you're installing a standalone into a new vehicle with all kinds of side / front / anus airbags, all kinds of engine sensors, safety equipment, this that and the other, you may want to go a more produced route. However, if you are stripping that same new vehicle to a track only car, then by all means, this would be a ECU kit to definately keep in mind.
I am god of the sea people.