<select name="author_id">
{exp:query sql="SELECT member_id AS member_author_id, username AS member_username, screen_name AS member_screen_name FROM exp_members"}
<option value="{member_author_id}" {if member_author_id == author_id}selected="selected"{if:elseif member_author_id == logged_in_member_id}selected="selected"{/if}>{member_screen_name}</option>
{/exp:query}
</select>
Via: http://expressionengine.com/forums/viewthread/184361/
If you use 0|{param} then if param is empty the tag will not show all entries.
{exp:channel:entries category="0|{embed:categories}" ....