<select name="taxonomy-dropdown" onchange="document.location.href=this.options[this.selectedIndex].value;">
	<option value="javascript:void(0)">Select</option>
	<?php dropdown_taxonomy_term('enter-taxonomy-name-here', 'orderby=name&order=asc&hide_empty=1&number=0'); ?>
</select>

<select name="speakers-filter" onchange="document.location.href=this.options[this.selectedIndex].value;">
	<option value="javascript:void(0)">Select</option>
	<?php dropdown_taxonomy_term('speakers', 'orderby=name&order=asc&hide_empty=1&number=0'); ?>
</select>

http://snipplr.com/view/41041/custom-taxonomy-terms-dropdown-function/

An example of how to call the dropdown taxonomy term function in the template.

Custom Taxonomy Terms Dropdown Template Tag

wordpress

PHP