<form name="pass_form">Пароль: <input name="pass" size="25" type="text" value="">
<input type="button" value="Сгенерировать" onClick="gen_pass()"></form>

<script type="text/javascript">
chr = new Array('A','B','C','D','E','F','G','H','I','J','K','L','M','N','O','P','Q','R','S','T','U','V','W','X','Y','Z',
'a','b','c','d','e','f','g','h','i','j','k','l','m','n','o','p','q','r','s','t','u','v','w','x','y','z',0,1,2,3,4,5,6,7,8,9);

function rnd(x,y,z) {
var num;
do {
num = parseInt(Math.random()*z);
if (num >= x && num <= y) break;
} while (true);
return(num);
}

function gen_pass() {
var pass = '';
for (var i = 0; i < 20; i++) pass += chr[rnd(0,61,100)];
document.pass_form.pass.value = pass;
}
</script>