jQuery is a fast and concise JavaScript Library that simplifies HTML document traversing, event handling, animating, and Ajax interactions for rapid web development. jQuery is designed to change the way that you write JavaScript.
it's lightweight (just 19 kb in size) CSS3 complient and with cross browser support (IE 6.0+, FF 2+, Safari 3.0+, Opera 9.0+, Chrome and almost all the browsers).
you can download the latest release from here