JsCalendar
This is the most popular DHTML Calendar. It can work as a popup calendar or a flat calendar (flat means it's always displayed in page). It works with Mozilla, Netscape >= 6.0, all other Gecko-based browsers, Internet Explorer >= 5.0 for Windows, Opera 7, Safari and Konqueror.
version 0.9.3 is bundled in tikiwiki 1.8 for use with tiki calendar and possibly many other forms implying dates selection. That feature is triggered by an option in features admin panel.
check http://dynarch.com/mishoo/calendar.epl for more info
Since version 0.9.6 there is a PHP wrapper, which also can be useful in tiki (quote from release notes):
Added a simple PHP wrapper. It provides code which loads all the required scripts and theme file, and one function which creates and configures an input field for date input. It takes care of creating and assigning unique ID-s for the calendar fields and it also creates the "Calendar.setup" code. Functions to create more specialized fields can be added very easily.
That way JsCalendar can be customized to display different themes and languages.