Klang offer a collection of free Kontakt instruments.
Native Instruments offer a ton of sounds (2000) and instruments (Synths, Smaples Instruments) plus an organizer app (Komplete Kontrol) for free. Download here.
Free Kontakt version of the Concertmate 300 (You might remember it from Casio or Tandy) for some Chiptune sounds. Get it from Sonus Instruments.
This might be the craziest library for Native Instruments Kontakt that you can find. It´s from Embertone and it´s free.