HTML:
<div class="hidebox">
Please leave blank: <input type="text" id="gotcha" tabindex="-1" name="gotcha" />
</div>

PHP:
<?php
if($_POST["gotcha"]!=""){
header("Location: {$_SERVER[HTTP_REFERER]}");exit;
}
?>

CSS:
.hidebox {
    position:absolute;
    background-color:#CC0000;
    left:0px;
    top:-500px;
    width:1px;
    height:1px;
    overflow:hidden;
}

Note for WordPress:
/wp-content/themes/theme-name/comment.php
add around line 65 (after author,email,url inputs):
<!--Catch sp@m-->
<p class="hidefield"><input type="text" name="gotcha" id="gotcha" size="22" tabindex="-1" />
<label for="gotcha"><small><?php _e('Leave blank!'); ?></small></label>
</p>

/wp-comments-post.php
Add after approx. line 54:
$comment_gotcha       = ( isset($_POST['gotcha']) ) ? trim($_POST['gotcha']) : null;

Add after approx. line 87:
if ( $comment_gotcha != '' ) die( __('Error: Your post has been rejected as spam') );	

i.e., after 

if ( '' == $comment_content )
	wp_die( __('Error: please type a comment.') );

/wp-content/themes/theme-name/style.css
.hidefield { position:absolute; background-color:#CC0000; left:0px; top:-500px; width:1px; height:1px; overflow:hidden; }

HTML:
<div class="hidebox">
Please leave blank: <input type="text" id="gotcha" tabindex="-1" name="gotcha" />
</div>

PHP:
<?php
if($_POST["gotcha"]!=""){
header("Location: {$_SERVER[HTTP_REFERER]}");exit;
}
?>

CSS:
.hidebox {
    position:absolute;
    background-color:#CC0000;
    left:0px;
    top:-500px;
    width:1px;
    height:1px;
    overflow:hidden;
}

HTML:
<div class="hidebox">
Please leave blank: <input type="text" id="gotcha" tabindex="-1" name="gotcha" />
</div>

PHP:
<?php
if($_POST["gotcha"]!=""){
header("Location: {$_SERVER[HTTP_REFERER]}");exit;
}
?>

CSS:
input#gotcha {
    position:absolute;
    background-color:#CC0000;
    left:0px;
    top:-500px;
    width:1px;
    height:1px;
    overflow:hidden;
}

http://www.bensky.co.uk/blog/code-tutorials/invisible-capcha-concept-stop-spam-email/

Create a textbox that the bot will presume is just a field it can exploit with rubbish and label it with an indicator to let any viewers without the privileges of CSS know that it is to be left blank.

Invisible Captcha

PHP