Zmax
I hear it advertised a lot, but have never used it.
Some shops here use BG Products engine/oil treatments (you can't buy it off the shelf, though) -- which are said to be very good.
My 2004 Silverado truck has 105,000 miles and I have had the fuel injection system, automatic transmission, power steering, coolant and rear differential all treated with the BG system (over the past two years). No problems with any of the aforementioned.
Since I bought it used in February of 2010 with 74,000 miles on it, I figured it might be a good idea for the long-term health of my truck.
:roadkill: