There are a number of complete engine control modules that completely replace the stock ECM.  The most common one in the STI world is Element Tuning’s Hydra EMS.  This is a very capable system with the ability to auto-tune itself in a closed loop fashion to make it hit your preset desired air fuel ratios.  Once the Hydra has finished auto-tuning itself and you’re satisfied, you can turn auto-tuning off and save the results to run in open-loop mode.  The Hydra does have some limitations in its current state of development.  Namely, it has an unwelcome quirk of slow starting, since it takes two revolutions of the crank for it to get a signal for spark timing.  The other big flaw is a lack of many functions that we have grown accustomed to, such as OBD2 functionality, Cruise Control (which is controlled by the ECM), and etc.  It has been said that Cruise Control can be added to the Hydra, but as yet, it is still not there.  Hydra is another not-cheap solution at a whopping $1650, which doesn’t even include the optional O2 sensor for the auto-tuning function.  Aside from these little niggles, the Hydra does offer the best feature set of all the common engine management systems, including launch control, pseudo-antilag, closed loop fuel control, high resolution mapping, rev limit, closed loop boost control, and so on.  It’s just that not being able to pass an OBD2 scan at the next emissions inspection is prohibitive for most owners.