<Select> User's own option

Does anyone know of a way I can let the user type an option (that isn't already an option) into the box.

cheers

Natively, only Gecko supports a native widget for it, but is only available in a XUL document, not an HTML or XHTML document.

There are numerous DHTML solutions though, such as:
http://webfx.eae.net/dhtml/combobox/combobox.htm