Here are the snippets of code I’ve managed to collect. If you have any WordPress code you’d like me to add, please leave a comment below!

Display Recent Posts

Here is the code you need to display the most recent 5 posts:

<?php query_posts('showposts=5'); ?>
<ul>
<?php while (have_posts()) : the_post(); ?>
<li><a href="<?php the_permalink() ?>"><?php the_title(); ?></a></li>
<?php endwhile;?>
</ul>

Display Recently Updated Posts/Pages

<?php
$today = current_time('mysql', 1);
$howMany = 5; //Number of posts you want to display
if ( $recentposts = $wpdb->get_results("SELECT ID, post_title FROM $wpdb->posts WHERE post_status = 'publish' AND post_modified_gmt < '$today' ORDER BY post_modified_gmt DESC LIMIT $howMany")):
?>
<h2><?php _e("Recent Updates"); ?></h2>
<ul>
<?php
foreach ($recentposts as $post) {
if ($post->post_title == '') $post->post_title = sprintf(__('Post #%s'), $post->ID);
echo "<li><a href='".get_permalink($post->ID)."'>";
the_title();
echo '</a></li>';
}
?>
</ul>
<?php endif; ?>

Display Recent Comments

<?php
global $wpdb;
$sql = "SELECT DISTINCT ID, post_title, post_password, comment_ID,
comment_post_ID, comment_author, comment_date_gmt, comment_approved,
comment_type,comment_author_url,
SUBSTRING(comment_content,1,30) AS com_excerpt
FROM $wpdb->comments
LEFT OUTER JOIN $wpdb->posts ON ($wpdb->comments.comment_post_ID =
$wpdb->posts.ID)
WHERE comment_approved = '1' AND comment_type = '' AND
post_password = ''
ORDER BY comment_date_gmt DESC
LIMIT 10";
$comments = $wpdb->get_results($sql);
$output = $pre_HTML;
$output .= "\n<ul>";
foreach ($comments as $comment) {
$output .= "\n<li>".strip_tags($comment->comment_author)
.":" . "<a href=\"" . get_permalink($comment->ID) .
"#comment-" . $comment->comment_ID . "\" title=\"on " .
$comment->post_title . "\">" . strip_tags($comment->com_excerpt)
."</a></li>";
}
$output .= "\n</ul>";
$output .= $post_HTML;
echo $output;?>

Display Top Comments

<?php $result = $wpdb->get_results("SELECT comment_count,ID,post_title FROM $wpdb->posts ORDER BY comment_count DESC LIMIT 0 , 10");
foreach ($result as $topten) {
$postid = $topten->ID;
$title = $topten->post_title;
$commentcount = $topten->comment_count;
if ($commentcount != 0) { ?>
<li><a href="<?php echo get_permalink($postid); ?>" title="<?php echo $title ?>"><?php echo $title ?></a></li>
<?php } } ?>

Display Categories

<h2>Categories</h2>
<ul>
<?php wp_list_cats('sort_column=name'); ?>
</ul>

Display Categories in Drop-Down Box

<form action="<?php bloginfo('url'); ?>/" method="get">
<?php
$select = wp_dropdown_categories('show_option_none=Select category&show_count=1&orderby=name&echo=0');
$select = preg_replace("#<select([^>]*)>#", "<select$1 onchange='return this.form.submit()'>", $select); echo $select; ?>
<noscript><input type="submit" value="View" /></noscript>
</form>

Display Archives

<h2>Archives</h2>
<ul>
<?php wp_get_archives('type=monthly'); ?>
</ul>

Display Archives in a Drop-Down Box

<select name=\"archive-dropdown\" onChange='document.location.href=this.options[this.selectedIndex].value;'>
<option value=\"\"><?php echo attribute_escape(__('Select Month')); ?></option>
<?php wp_get_archives('type=monthly&format=option&show_post_count=1'); ?> </select>

Display a Pages Menu in your Sidebar

<h2>Pages</h2>
<ul>
<?php wp_list_pages('title_li='); ?>
</ul>

Display Gravatars (WordPress 2.5+ Only)

<?php if(function_exists(’get_avatar’)){ echo get_avatar($comment, ‘50?);} ?>

Display Blogroll Links

<ul>
<?php get_links_list(); ?>
</ul>

Display Admin Section

<ul>
<?php wp_register(); ?>
<li><?php wp_loginout(); ?></li>
<li><a href="http://www.wordpress.org/">WordPress</a></li>
<?php wp_meta(); ?>
<li><a href="http://validator.w3.org/check?uri=referer">XHTML</a></li>
</ul>

Display a Pages SubMenu in your Sidebar

This will display any subpages in your blog’s sidebar:
<?php$children = wp_list_pages('title_li=&child_of='.$post->ID.'&echo=0');if ($children) { ?><ul> <?php echo $children; ?>
</ul>
<?php } ?>

Display WordPress Tags

<?php the_tags(); ?>

Display WordPress Tags Cloud

<?php wp_tag_cloud('smallest=8&largest=36&'); ?>

Template Name

This allows you to use the WordPress page template to customize how a page is displayed:
<?php /* Template Name: Portfolio */ ?>

Dynamic Title Tags

<title><?phpif (is_home()) { echo bloginfo('name');
} elseif (is_404()) {
echo '404 Not Found';
} elseif (is_category()) {
echo 'Category:'; wp_title('');
} elseif (is_search()) {
echo 'Search Results';
} elseif ( is_day() || is_month() || is_year() ) {
echo 'Archives:'; wp_title('');
} else {
echo wp_title('');
}
?></title>

Display PHP on a Single Page

Allows you to display plugins and such on a single page (replace home with the page you want it to only appear on):

<?php if ( is_home() ) { include ('file.php'); } ?>

Display an External RSS Feed

<?php include_once(ABSPATH.WPINC.'/rss.php');
wp_rss('http://wpforums.com/external.php?type=RSS2', 5); ?>

Display Most Recent Twitter Entry

<?php
// Your twitter username.
$username = "TwitterUsername";
// Prefix - some text you want displayed before your latest tweet.
// (HTML is OK, but be sure to escape quotes with backslashes: for example href=\"link.html\")
$prefix = "";
// Suffix - some text you want display after your latest tweet. (Same rules as the prefix.)
$suffix = "";
$feed = "http://search.twitter.com/search.atom?q=from:" . $username . "&rpp=1";
function parse_feed($feed) {
$stepOne = explode("<content type=\"html\">", $feed);
$stepTwo = explode("</content>", $stepOne[1]);
$tweet = $stepTwo[0];
$tweet = str_replace(”&lt;”, “<”, $tweet);
$tweet = str_replace(”&gt;”, “>”, $tweet);
return $tweet;
}
$twitterFeed = file_get_contents($feed);
echo stripslashes($prefix) . parse_feed($twitterFeed) . stripslashes($suffix);
?>

I’m expecting this list to grow over time as I come across more useful WordPress code and update the post.