Virgin Choir is a free VST plugin that recreates the sound of a female choir. It is a very high quality HD sounding plugin with features like multiple articulations with easy keyswitching, velocity sensitivity for the choir singing, and even a sequencer mode. It is pretty rare to find a 100% free VST plugin that sounds so good and looks so good.
It definitely takes some creativity in your sound design and composition skills to use choir sounds in your beats but when you got one that is calling for some choir runs then definitely give Virgin Choir a shot.
As of right now Virgin Choir is for PC only so mac users gotta take the L on this one 😢