Dropdown sidebar filter by category?

You could possibly use query_posts() – http://codex.ww.wp.xz.cn/Function_Reference/query_posts, but I’d use a custom query.

I’m assuming you want the results to be displayed on your index page and that you want only the Post Title to be searched? If so, add this to the top of your index.php page –

global $wpdb;
$posts = $wpdb->get_results($wpdb->prepare(
	'SELECT * FROM '.$wpdb->posts.'
	WHERE post_title="$_GET['s']"
	OR post_title="$_GET['+']"
'));

It’s probably because you are using the _GET method rather than the _POST method – I’d advise changing that and then amending the code I gave to this –

global $wpdb;
$posts = $wpdb->get_results($wpdb->prepare(
	'SELECT * FROM '.$wpdb->posts.'
	WHERE post_title="$_POST['s']"
	OR post_title="$_POST['+']"
'));

To be honest, I’d also recommend changing the name field of your <select> boxes, as ‘s’ and ‘+’ don’t really mean much. You can literally name them what you want.

Sorry, that’s me typing badly – should be this…

global $wpdb;
$posts = $wpdb->get_results($wpdb->prepare(
	'SELECT * FROM '.$wpdb->posts.'
	WHERE post_title="'.$_POST['s'].'"
	OR post_title="'.$_POST['+'].'"
'));

Ok, I see what you are looking to happen, but I’ve gone in a slightly different direction. The _POST method is much more scalable than the _GET method, so I’d abandon that. Change your original code that you posted to –

<form method="post" id="searchform" action="<?php bloginfo('siteurl'); ?>">
  <div>
    <label class="screen-reader-text" for="s">Search for:</label>
Type
<select name='field1' id='field1' class='postform' >
  <option value='0' selected='selected'>All</option>
  <option class="level-0" value="dog">dog</option>
  <option class="level-0" value="cat">cat</option>
</select>	

Colour
<select name='field2' id='field2' class='postform' >
  <option value='0' selected='selected'>All</option>
  <option class="level-0" value="white">white</option>
  <option class="level-0" value="black">black</option>
</select>	

   <input type="submit" id="searchsubmit" value="Search" />
  </div>
</form>

Then, as a starter, add this to the top of your index page (to make sure the correct results are getting through –

echo '<pre>';
print_r($_POST);
echo '</pre>';

If you have the correct options that you selected for your search in the _POST array, then replace the above code with the code I supplied earlier –

global $wpdb;
$posts = $wpdb->get_results($wpdb->prepare(
	'SELECT * FROM '.$wpdb->posts.'
	WHERE post_title="'.$_POST['field1'].'"
	OR post_title="'.$_POST['field2'].'"
'));