Kontakt is described as 'KONTAKT 5 takes a bold leap forward with 37 new filters, four new on-board studio effects, updated time-stretching, a new instrument bus system and more' and is an app in the Audio & Music category. There are more than 10 alternatives to Kontakt for a variety of platforms, including Windows, Mac, Linux, BSD and iPad. The best alternative is Carla, which is both free and Open Source. Other great apps like Kontakt are LinuxSampler (Free, Open Source), TX16Wx (Freemium), Grace Sampler (Free) and UVI Falcon (Paid).
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.
The intersection of sampling and synthesis. Falcon is a virtual instrument with 15 oscillators including analog, FM, wavetable, granular, … , 80+ effects, powerful modulation generators, scriptable event processors and more. Load as a stand-alone software or a DAW plug-in.
VSampler transforms your PC into a 255 voice polyphonic software sampler that is able to use any recorded sound as a musical instrument. It provides all of the classical sampler features in excellent sound quality, and also offers you a multitude of unique new opportunities.
Pianoteq is a virtual piano that runs on your computer. Celebrated for reproducing with an unprecedented realism the behaviour and the sound of the acoustic piano, it offers superb dynamics, natural resonances and unique physical parameters.