c2ctl is a frequency and voltage modification utility mainly intended for Intel Core 2 and Core CPUs. It should also work for other CPUs (namely processors with SpeedStep). The utility can also enable SpeedStep (e.g. if it was disabled by the BIOS due to overclocking) and read out frequency and voltage information. The main purpose of c2ctl is overclocking or "undervolting" and to enable SpeedStep.
k10ctl is a P-State utility for AMD family 10h (aka K10) processors. It can modify the P-States, i.e. alter processor and the northbridge frequencies and voltages, set or change the P-State, and read out frequency and voltage information. The main purpose of k10ctl is overclocking or "undervolting" the CPU and to configure Cool'n'Quiet settings.