Zepto is a nice alternative, with a jQuery-compatible API and a way smaller source. It would be the perfect replacement for jQuery if it wasn't for it's lack of IE support.
For that reason I and a friend started a project called Rye[1] that is a light wrapper around the DOM and other browser APIs; it's made of small modules that you can pick as needed. Despite being < 6kb, it also favours readability over code size, so it's easy to spy under the hood.
Ender[2], a hybrid library/package manager, has a similar philosophy.
[1] http://ryejs.com