Revison 1
Essentially, lets your users click a whole line of checkboxes by clicking the first and dragging down the line.

This is a crop-in script. That is to say, include it on your page, and you need do nothing else. Even primitive event handlers (element.onchange, for example) are supported, with no action on your part.

A demo of it in action is on my hosting:
http://fordi.org/test/checks.php

This is the libless version; that is to say, it is independent of and doesn't interfere with any Javascript libraries, such as jQuery, Prototype, MooTools, etc.