Save your money. Most additives don't help your car much, despite their advertising or other claims. Some you might need for specific uses -- say, Dry Gas in the winter, or a fuel stabilizer if you're storing an engine for months -- but most of the engine treatment stuff is expensive and doesn't really do anything.
All those teflon treatments -- Greased Lightning, Slick 50, etc. -- all crap.
Always check the Material Safety Data Sheets for anything you buy, and I'll guarantee you won't see any miracle ingredients in any auto additives. (Seafoam, btw, is just a light machine oil, naptha and isopropyl alchohol, better known as rubbing alcohol. The IPA probably helps get water out of the gas, and the naptha likely helps desludge carbs, but you could make up a similar solution in your garage....)
The best additive for your engine is a quality oil, changed often.....Check out www.bobistheoilguy.com for some serious data....
EDIT: Steve, you slapped me with a memory stick....The main case for an additive might be if you were having some upper valvetrain noise., and I remember my 86 C900T started to get a little talkative at 120K miles or so. My dad added a pint of Marvel and it helped, but I was advised to change the oil quickly.
I have used Marvel Mystery Oil in my C900 and it really quieted the motor down from how it sounded when I got it. I agree with Mike here about oil changes being the best thing you can do to keep the longevity of the motor. For our cars with turbos the best way to go is a full synthetic oil in my opinion as it doesnt break down in high heat (as much) and will reallly help your car to stay lubed.
I have heard it doesnt really matter what you use as far as oil goes. The most important thing however, is you find one and stick to it. If it is a new car go full synthetic but still change regularly, just because synthetic doesnt brake down as fast as conventional oil doesnt mean you should wait 10K miles. People have told me before that I am full of %&*# but it is just what I have heard.
I am a big fan of Chevron Techron Complete Fuel System Cleaner. I treat it about every 3,000 miles per the directions. I use 3 oz. Fuel Power with every fill up. It gets a lot of good pub on Bobistheoilguy.com. No additives for oil or transmission. I use Redline 5W40 and Mobil 1 or Redline ATF. For Coolant, I've gone to Zerex G-05 (yellow).
I use full synthetic in my C900 and it made it run so much quieter than regular oil. I would not advocate not doing 3000 mile oil changes either just because you run Synthetic oil.
I do also run Watter Wetter in my coolant to help keep things a little lower temped. It seems to reduce my coolant temps a bit and help the car to disipate heat better.