Pianobook is a community-driven library created by Christian Henson (Spitfire Audio). Here, musicians sample their own pianos, synths, and found sounds.
Production Voices created a stripped-down version of their "Production Grand" piano. The is a free, playable piano that runs perfectly in Kontakt 5 Player. It is a staple for pop producers.
Runs all NI libraries and many third-party libraries for free. It has limitations on editing complex instruments and only runs libraries that are "licensed" or "powered" for Player.
Native Instruments includes a free bundle of sounds (Kontakt Factory Selection) to get you started immediately. Top Free Libraries for Kontakt Player