jdPicker- The Ultimate jQuery Date Picker
A very nice calendar that you can integrate in your site is the jdPicker. It is a free, non-obstructive, and highly customizable jQuery plugin that lets you add a folding calendar. Its original version comes in complete English and the creators are working on other language translations.
It works with an American Date Format (YYYY/mm/dd) but you can select from four available formats and you can also add your own by simply editing few lines in the script.
Click on the image to view its full documentation, about/license, translation, live demo and download source.