Theme the TinyMCE Editor

Loads a CSS file inside the editor iFrame so your preview of the content can be styled to look like the output in your theme. Just add custom-editor-style.css in your theme folder

Post/Page Thumbnails

Enable the thumbnails functionality for your theme.

Theme the login page

Loads a CSS file into your login page so you can override default styles. Just add wp-login.css into your theme folder.

Register wigetable sidebar

Let your users put loads of crap into the sidebar! You may need to tweak your theme files, read this article.

Remove Admin Bar

The admin bar on the frontend can be a bit of a pain if your using absolute / fixed positioning, or maybe you just hate it? Let's get rid of it.

Customise Admin footer

Change the message that appears in the admin footer, why not give yourself a credit for the theme?

Set max-width for Oembed objects

Define a max-width for Oembed objects such as Youtube videos.

Add posts & comments RSS feeds

Adds RSS feeds to your sites header.

Post thumbnail in RSS Feed

Adds in the post thumbnail to your RSS feed. Submitted by Claudio Schwarz

Theme the admin area

Loads a CSS file into your admin area so you can override default styles. Just add wp-admin.css into your theme folder.

Custom Backgrounds

Let your users change the themes background image through 'appearance - backgrounds'.

TinyMCE Placeholder Text

Add an array of placeholder messages to the edit screen of posts or pages. Perhaps a friendly message or a reminder of how to do something. Via Ant Januska

Get more search results

By default, your search page respects the 'posts per page' set in the reading section of admin settings. Let's override that to get as many as you like.

Custom Excerpt Permalink

Removes the standard elipsis (...) that appears at the end of the_excerpt(); and replaces it with a custom message and permalink.

Add body class if there's a sidebar

Adds a has_sidebar class to the body if there's a sidebar. Use with body_class(); in your header.php file. Submitted by Andy Warburton.

Custom Thumbnail Sizes

Create a selection of custom image sizes you can pull through in your template like this: <?php the_post_thumbnail('mySize'); ?>

Separate Images & Paragraphs

Stop images getting wrapped up in paragraph tags when they get dumped out with <?php the_content(); ?>

Use Googles copy of jQuery

Grabs jQuery from Googles CDN which your visitors will hopefully have cached. This can then be called in your header with <?php wp_enqueue_script("jquery"); ?>

Custom Excerpt Length

Define how many words to return when using the_excerpt();.

HTML5 Shim only for IE

Calls Googles copy of the HTML5 shim/shiv for users on IE and only loads it if they're using IE8 or below.

Remove WP version number

Removes the version number from your header so attackers can't target known weakness's. Remember to also remove the readme.html file in your root directory which also contains this information.

Obscure Login Error Messages

Don't let people know which is incorrect: the username or password.

Disable Admin Theme Editor

Removes the code editor in the admin which allows anybody with access to modify the theme and plugin files.