Avgrund.js is a jQuery plugin for modal boxes and popups. It uses interesting conceptshowing depth between popup and page.
It works in all modern browsers and gracefully degrade in those that do not support CSS transitions and transformations (e.g. in IE 6-9 has standard behavior). It has been tested with jQuery 1.4+. And it is lightweight, file size is under 2Kb. It is licensed under MIT License.