This is a JavaScript pop-up calendar, and it ROCKS.
I take no credit for it whatsoever. The code here is all Matt Kruse', and was copied from his site purely to help illustrate the many different tweaks available to display his calendar. Here's the kewl guy who is responsible: http://www.mattkruse.com/javascript/calendarpopup/.

(View Source of this page to see the example code)
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Default calendar.
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Default calendar using the DIV-style display.
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Default calendar using the DIV-style display, with navigation drop-downs enabled.
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DIV-style calendar using a CSS prefix and different styles define in this HTML page (view source to see the defined styles)
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Some dates manually disabled from selection.
Dates disabled: Anything up to today, December 25, 2007, and anything after January 1, 2008.
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Week-end select.
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Calendar with showYearNavigation() enabled.
select    clear date    (LINK WHICH CLEARS THE DATE ENTERED WHEN CLICKED)
Calendar with showYearNavigation() enabled and showYearNavigationInput() enabled, to allow manual entering of years
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Calendar with only Saturdays allowed to be selected enabled.
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German Calendar, with modified month names, day names, and week starting on Monday. Date format changed to dd/MM/yyyy
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Month-select calendar
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Quarter-select calendar
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Year-select calendar
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Default calendar, but results are split into multiple fields.
/ / (m/d/y) select
/ / (mm/dd/yyyy) select
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Calendar with popup pre-selected to be January 29, 1974 (Matt's birthday!)
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Start date and end date, with end date popup defaulting to same date as start date
Start: select     End: select
Start: / select     End: / select