Rift energy was energy derived from temporal rifts. The Doctor used the Cardiff Rift to refuel his TARDIS (TV: Boom Town, Utopia)
In an alternate timeline created by John Hart, he used distilled rift energy with technology from the Torchwood Institute to cut Jack Harkness off from the Time Vortex, nullifying his immortality and transfering it to himself. Jack, however, remained a fixed point in time, and his eventual death resulted in John's universe collapsing in on itself. (AUDIO: The Death of Captain Jack)