TwentyBot

function countPieces() {
    it = b.pieces();
    var i = 0;
    while (it.current()){ i++; it.next() }
    return i;
}

function getRandPiece() {
    var it = b.pieces();
    var n = Math.floor(Math.random()*countPieces());
    for (var i = 0; i < n; i++){ it.next(); }
    return it.current();
}

function stackRandPiece(){
    if (b.isBusy()) { return; }
    if (b.isGameOver()) { clearInterval(randInterval); return; }
    
    var p = getRandPiece();
    b.setTarget({x: p.pos.x + 100, y: 1500});
    b.grab(p);
    
    window.setTimeout(function(){
        b.setTarget({x: (Math.floor(Math.random()*7)*200)+100, y: 1500});
        window.setTimeout(function(){b.release();}, 15);
    }, 15);
}

randInterval = window.setInterval(stackRandPiece, 40);