Our friends at WavesFactory, makers of the awesome TrackSpacer, have created a free Kontakt library of guitar power chords.
Power Guitars Free is a library for Kontakt featuring electric guitar power chords muted and sustained. Power Guitars Free is perfect to play quick background electric guitar rhythms to any rock/heavy/metal track. The samples have been recorded chromatically (no pitch-shifting, all chords sampled), with 2 round robin (repetitions) per chord. This is the free version. You only have 2 round robin and you get distorted guitar sound (no direct input).
The Pro version gives you 8 round robin and direct input so you can use your own amp simulation software to customize the sound.