The greatest common factor (a.k.a. GCF or greatest common divisor) is the largest number that will divide two or more numbers exactly. For example, the GCF of 9, 12, and 15 is 3.
Students often find LCMs and GCFs when working with fractions. To practice finding greatest common factors and least common multiples, Brian Schau has created LcmGcf for Palm computers. It's based on MathAce, so you are probably familiar with its interface and options. In LcmGcf you practice solving LCMs and GCFs of two or three numbers. You adjust the settings of the quiz, take the quiz, and then enter the score into the Last 10 Scores list. Also, the settings can be beamed from one handheld to another, just like MathAce. LcmGcf's User Manual gives some basic directions for using the software.