Hello,
Today I’ve been working on a related items section for wordpress posts. I found some really useful resources (see the end of the post). And set to work. The result is below:

Related Items Box From Style Compare

The function code

function getRelatedPosts($num, $imgWidth, $imgHeight, $className, $boxTitle) {
        
        // store category objects for current post in an array
	$current_post_categories = get_the_category(); 
	
	$catArray       = array(); // Initiate empty category array
	$posts_array 	= array(); // Initiate empty posts array
	
	$numberOfPosts = 0;
	
	// Puts category IDs into the $catArray
	foreach( $current_post_categories as $category) { 
		array_push($catArray, $category->cat_ID);
	}
	
	// I'm getting a random catID from my array, 
        // if there's only one caetgory we'll just get that
	for ($i=0; $i < $num; $i++) {	
	        
                // -1 because arrays are zero based
		$randomNum = rand(0, (count($catArray)-1)); 
	
		$args = array(
			'numberposts'     => 1,
			'category'        => $catArray[$randomNum],
			'orderby'         => 'post_date',
			'order'           => 'DESC',
			'offset'	  => $i, // add offset so we don't get 
                                                 // the same record again and again
			'exclude'         =>  get_the_ID(), // exlude the current post
			'post_type'       => 'post',
			'post_status'     => 'publish' 
		);		
			
		$posts = get_posts( $args );
		
		if ($posts[0] != "") {
                        // put the post array into a new array
			array_push($posts_array, $posts); 
		}
	}	
	
	$currentPostNumber = count($posts_array);
	$postsLeft = ($num - $currentPostNumber);
	$authorID	= get_the_author_meta();	
	
	// If we still haven't got enough, 
        // let's get the most recent author posts and 
	// stack um high so we have loads to choose from, better safe than sorry
	if ($currentPostNumber < $num) {
		
		for ($i=0; $i < $num; $i++) { 
		
			$args = array(
				'author'	  => $authorID,
				'numberposts'     => 1,
				'offset'	  => $i,
				'orderby'         => 'post_date',
				'order'           => 'DESC',
				'exclude'         =>  get_the_ID(), 
                                // exlude the current post
				'post_type'       => 'post',
				'post_status'     => 'publish' 
			);	
			
			$posts = get_posts( $args );
			
			array_push($posts_array, $posts);
		}
	}
	
	// Let's build our related item's box
	echo '<div class="' . $className . '">';
	echo '<h2>' . $boxTitle . '</h2>';
		echo '<ul>';
	
			for ($i=0; $i<$num; $i++) {
				
				$postID = $posts_array[$i][0]->ID;
				$desc  = $posts_array[$i][0]->post_content;
				
				$first_img = '';
				$output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $desc, $matches);
				$first_img = $matches [1] [0];
				
				if(empty($first_img)){ 
                                        //Defines a default image
					$first_img =  "/images/Default-Post-Image.jpg";
				}
				
				$url   = get_permalink($postID);
				$title = $posts_array[$i][0]->post_title;
				
				echo '<li><a href="'. $url .'"><img title="' . $title . '" src="' . $first_img . '" width="' . $imgWidth . 'px" height="' . $imgHeight . 'px"><h3>' . $title . '</h3></a></li>';
			}
	
		echo '</ul>';
	echo '</div>';
}

The CSS for the div

.relatedPostsBox {
	 display: block;
	 border-bottom: dotted 1px #ccc;
	 margin-top: 20px;
	 margin-bottom: 20px;
	 font-family:Tahoma, Geneva, sans-serif;
	 padding-left: 20px;
	 padding-bottom: 20px;
}

.relatedPostsBox h2 {
	 margin-bottom:20px;
}

.relatedPostsBox h3 {
	 font-family:Tahoma, Geneva, sans-serif;
	 font-size:12px;
	 padding:0;
	 margin:0;
}

.relatedPostsBox ul {
	 display:inline-block;
	 list-style:none;
	 list-style-type:none;
	 margin:0;
	 padding:0;
}

.relatedPostsBox li {
	 width: 140px;
	 display:inline-block;
	 float: left;
	 margin-right: 5px;
	 border: 1px #EFEFEF solid;
	 padding:5px;
	 overflow: hidden;
	 min-height:200px;
	 margin-right: 10px;
}

.relatedPostsBox ul img {
	 display: inline;
	 margin-bottom: 10px;
}

.relatedPostsBox ul h3, .relatedPostsBox ul p {
	 width: 140px;
}

.relatedPostsBox ul h3 {
	 margin:0;
	 font-size:11px;
	 line-height:15px;
	 font-weight:bold;
}

.relatedPostsBox ul p {
	 margin:0;
	 font-size:11px;
	 line-height:15px;
}

.relatedPostsBox ul a {
	 text-decoration: none;
	 color: #333;
}

.relatedPostsBox li:hover {
	 border: solid 1px #ccc;
}

Resources:
http://www.daniweb.com/web-development/php/threads/265806/get-the-category-link-inside-the-loop-in-wordpress
http://www.wprecipes.com/how-to-get-the-first-image-from-the-post-and-display-it