Incredibox

Assign a bass line, vocal loop or percussive sound to an animated beatboxer, then hear how the combination changes when someone drops out. The loops already fit together, leaving room to practise arranging without a test of fast fingers. You can record a mix when it finds a shape you like. The free browser demo offers a taste, while the paid app contains more styles. It is a playful musical construction kit rather than a full recording studio.