jquery.tristate.js is a jquery plugin for adding "tristate" indeterminate functionality to checkboxes in nested lists. 

To use it, simply call it on your parent list:
$('ul.tristate').tristate();

By default, the inputs must be direct descendants of the list elements. You may optionally pass in an array of wrappers that represents the tree of elements the inputs are nested within, e.g.:
$('ul.tristate').tristate({wrappers:['span.classname','label']});

Example available here:
http://jlbruno.github.com/jQuery-Tristate-Checkbox-plugin/