For my track car I wanted to have more control over the engine. A programmable ECU would open the road towards increasing the power output of the engine through forced induction. I spent some time looking at MegaSquirt, but then I found Speeduino, and I assembled one of the prototype boards for relatively little money.
I also found the connector that the stock ECU uses, and added that to the speeduino case. Now all it needs is to be wired up and installed in the car.