The MooTools DatePicker from MonkeyPhysics is extremely simple to implement, works beautifully and is extremely customizable. On the site, you can also download four different “skins” for the date picker. For my purposes, I went ahead and stuck with the default skin (which only requires the download of a CSS file), but I can certainly see the value in some of the other themes. The page includes a lot of nice examples and some great documentation. In addition to what’s provided on that site, I’ve added a handful of tips after the jump.