I am developing an in-house PHP leave scheduler and needed a datepicker /calendar control.
My Google searches brought me to Vista-like AJAX Calendar Version 2 and I must say its by FAR the best one if have ever come across. Looks amazing, very easy to use and extremely customizable, even for a PHP beginner such as myself. Well done to the developers!!!
One thing I have to mention, initially I ran across a problem where the dates did not display correctly. This was caused by incorrect IIS user rights, so if you run into similar problems just make sure your rights are correct (iwam/iusr).
Oh and please make sure you have declared the html namespace because it changes the way your browser renders your page. To be very honest I have never paid much attention to it, untill this morning when I realised IE7 is way more sensitive than Chrome!!!