Not many people know about this. In a HTML dropdown (select) there is a option group tag that groups your options together. For instance:

<select name="cars">
	<optgroup label="Honda">
		<option>Honda Accord DX</option>
		<option>Honda Accord LX</option>
		<option>Honda Accord EX</option>
	<optgroup label="BMW">
		<option>BMW 530iL</option>
		<option>BMW 640iL</option>
		<option>BMW 740i</option>

If you run the code above you will notice users can’t even select the option group (which is perfectly valid). And the automatic indendation within the sub-groups is very useful also. This feature is supported by Opera and IE browsers. The actual version details you will have to find out.