Is it possible using html or javascript, to allow users to type in a drop-down box if the option they're after doesn't exist in the list of options? I've defintely seen this before at least in a windows applications, and I'm pretty sure I've seen it online as well.

Does anyone know if this is at all possible? The only alternatives to this is to either have drop-down menu AND a textfield (wastes space), or have a little button which switches between the drop-down menu and a textfield. Both of those alternatives aren't ideal.