I want to be able to make an HTTP call updating some select boxes after a date is selected. I would like to be in control of updating the textbox so I know when there has been a "true" change (in the event the same date was selected). Ideally, I would call a function to pop-up the calendar and be able to evaluate the date before populating the text box...so I can do my validation before making a server call.
I've been playing with the jquery datePicker script - you should be able to do everything you need to with this.
I don't like the MS ASP.NET ajax, but their datepicker is superb. Otherwise, jQuery datepicker.
If you ever end up considering a JavaScript library/toolkit, Dijit, a widget system which layers on top of Dojo, has a calendar (Dijit calendar test page). I found it relatively simple to implement.
//Disclaimer: I'm in the middle of a love-hate relationship w/ Dojo at the moment, as I am in the process of learning and using it better.
Check out the ASP.NET AJAX Calendar Extender or Steve Orr's drop down Calendar control.
JQuery's datepicker is an extremely flexible tool. With the ability to attach handlers prior to opening or after date selection, themes, range selection and a variety of other incredibly useful options, I've found that it meets all my needs.
The fact that I sit next to one of its maintainers here at work is also fairly useful...
I'm using the JSCalendar from Dynarch for a project I'm currently working on. It is LGPL licensed and really flexible (easy to customize to your needs). It has lots of features and looks good, too.