Featherlight is a very lightweight jQuery lightbox plugin. It's simple yet flexible and easy to use. Featherlight has minimal css and uses no inline styles, it offers image, ajax and iframe support out of the box.
Serenade.js is a client side framework built on the MVC pattern. It makes it simple to create rich client side applications by freeing you from having to keep the DOM up to date with your data through powerful data bindings.
Small & fast DOM and event library for modern browsers. Having the same familiar API as jQuery (but without the extra "weight" of modules like $.ajax, $.animate, and $.Deferred), it works great stand-alone or paired up with e.g. Backbone.