megasquirt is a diy standalone fuel and spark computer. its pretty powerful, can run MAP/MAF, alpha-n or hybrid alpha-n (like the ferrari f40!) and be configured to run just about any setup under the sun.
its not anywhere near as straightforward to install as units like gunni uses, but for me the benefits (features and cost) just about outweigh that downside. like i've said though, the jury's still out on it for me until i've been running the unit for a good while.
in brief pros are cost, features, cost, support community and wide range of open-source software, also very fast development etc. oh, and did i mention cost?
cons are potentially reliability (though most squirters will say thats crap!), difficult to setup and install - much more involved, has to be built, and requires a lot more knowledge from the installer. and there's a lot of configuration too.
hth
tim