The Apollo Lunar Module (LM) descent engine, the all-time throttling champ, did it perfectly on six landings in 1969-72. It could throttle from 10,125 lbs down to 1,250 lbs. It was also a simple engine, burning corrosive fuel and oxidizer that ignited on contact, and fed by pressurized tanks, eliminating the need for pumps.

NASA is heading back to the Moon in the next decade, and "we want to put more mass down on the lunar surface than Apollo did. That means we need a higher-performing engine," says engineer Tony Kim of NASA's Marshall Space Flight Center. "The Apollo Lunar Module descent engine was very good, very reliable, but it doesn't have the performance we need for future exploration."

http://science.nasa.gov/headlines/y2007/16jul_cece.htm