jQuery UI lets you build a lot of cool elements with minimal work. Things like dropdown menus and date pickers (and much more!). See http://jqueryui.com/ for more. You can 'roll your own' theme, or pick from thousands of premade ones. http://jqueryui.com/themeroller/ (A theme is like a color scheme). Some examples below.

jQuery UI is a separate library that works with jQuery. To include it, add the following lines to the head of your page AFTER you include jQuery: 

<!-- Include jQuery UI - also hosted at Google -->
<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jqueryui/1.8.0/jquery-ui.min.js"></script>
<!-- jQuery UI css file -->
<link rel="stylesheet" href="http://jquery-ui.googlecode.com/svn/tags/latest/themes/base/jquery-ui.css" type="text/css" media="all" />

Date Picker

Slider

Check out my GIANT slider!

Date Picker: http://jqueryui.com/demos/datepicker/

Slider: http://jqueryui.com/demos/slider/