Check all other checkboxes when one is checked

demo

updated_demo

HTML:

<label><input type="checkbox" name="sample" class="selectall"/> Select all</label>

<div id="checkboxlist">

    <label><input type="checkbox" name="sample[]"/>checkbox1</label><br />
    <label><input type="checkbox" name="sample[]"/>checkbox2</label><br />
    <label><input type="checkbox" name="sample[]"/>checkbox3</label><br />
    <label><input type="checkbox" name="sample[]"/>checkbox4</label><br />

</div>

JS:

$('.selectall').click(function() {
    if ($(this).is(':checked')) {
        $('div input').attr('checked', true);
    } else {
        $('div input').attr('checked', false);
    }
});

HTML:

<form>
<label>
    <input type="checkbox" id="selectall"/> Select all
</label>
<div id="checkboxlist">
    <label><input type="checkbox" name="sample[]"/>checkbox1</label><br />
    <label><input type="checkbox" name="sample[]"/>checkbox2</label><br />
    <label><input type="checkbox" name="sample[]"/>checkbox3</label><br />
    <label><input type="checkbox" name="sample[]"/>checkbox4</label><br />
</div>
</form>

JS:

$('#selectall').click(function() {
    $(this.form.elements).filter(':checkbox').prop('checked', this.checked);
});

http://jsfiddle.net/wDnAd/1/

Thanks to @Ashish, I have expanded it slightly to allow the "master" checkbox to be automatically checked or unchecked, if you manually tick all the sub checkboxes.

FIDDLE

HTML

<label><input type="checkbox" name="sample" class="selectall"/>Select all</label>

<div id="checkboxlist">
    <label><input type="checkbox" class="justone" name="sample[]"/>checkbox1</label><br/>
    <label><input type="checkbox" class="justone" name="sample[]"/>checkbox2</label><br />
    <label><input type="checkbox" class="justone" name="sample[]"/>checkbox3</label><br />
    <label><input type="checkbox" class="justone" name="sample[]"/>checkbox4</label><br />
</div>

SCRIPT

$('.selectall').click(function() {
    if ($(this).is(':checked')) {
        $('input:checkbox').prop('checked', true);
    } else {
        $('input:checkbox').prop('checked', false);
    }
});

And now add this to manage the master checkbox as well...

$("input[type='checkbox'].justone").change(function(){
    var a = $("input[type='checkbox'].justone");
    if(a.length == a.filter(":checked").length){
        $('.selectall').prop('checked', true);
    }
    else {
        $('.selectall').prop('checked', false);
    }
});