The best free alternative to Kontakt is Carla, which is both free and Open Source. If that doesn't suit you, our users have ranked more than 10 alternatives to Kontakt and seven of them is free so hopefully you can find a suitable replacement. Other interesting free alternatives to Kontakt are LinuxSampler (Free, Open Source), TX16Wx (Freemium), Grace Sampler (Free) and Soundplant (Freemium).
Carla is an audio plugin host, with support for many audio drivers and plugin formats. It has some nice features like automation of parameters via MIDI CC (and send output back as MIDI too) and full OSC control.