The LS1M (LS1M) if you can find a good one used. You can't buy new anymore. I've heard Autotap, but I have no experience with it, personally. My favorite is the HPTuners but, it's not for the budget minded. Then you could always make friends with a local GPer.
One thing to note: A scanner seems a like a huge investment, but if it keeps you from blowing an engine, you are way ahead of the game.