This is a simple Javascript snippet that lets you toggle a group of checkboxes on or off at the same time. This is the format for your HTML form:

<input type=checkbox id='toggleall' onClick="toggleAll('myform');"> Toggle All

<input type=checkbox id='myform1'>
<input type=checkbox id='myform2'>
<input type=checkbox id='myform3'>

You can use this in multiple groups on the same page or form. Groups are determined by the checkbox id's which need to match followed by 1, 2, 3..

function toggleAll(id) {
    box = document.getElementById(id);

    if(box.checked == true) var setting = true;
    else var setting = false;

    var cur_id = id+'1';

    var count = 1;
    while(box = document.getElementById(cur_id)) {
        box.checked = setting;
        cur_id = id+count;
        count++;
    }
}