Uniform is a jQuery plugin that enables you to control the look & feel of form elements (checkboxes, drop down menus, radio buttons & file upload inputs).
It is unobtrusive & works by hiding the original elements + replacing them with custom ones.
Uniform is very easy to use & can be set to convert every form element on the page or any specific ones.
The plugin has theming support & comes with a theme kit to quickly create unique forms.
Requirements: jQuery 1.3+
Compatibility: All Major Browsers (except IE6)