Kalendae attempts to make a date picker that does not suck. It is fully portable and does not need external dependencies. Does not require jQuery, it is easily skinnable. The default theme uses only one image file (a mask for the previous and next buttons), and everything else is styled using CSS.
This calendar supports all modern browsers and IE8, single day, multiple day, or day range selection, the number of months to be displayed at once can be configurable; it can be displayed on the page as an inline widget, or attached to one or more input fields as a pop-up control, among many other features.