Best collection of code for your 'functions.php' file

Modify the Login Logo & Image URL Link

Tested on: WordPress 3.0.1

This code will allow you to easily modify the WordPress Login page Logo as well as the href link and title text of this logo.

add_filter( 'login_headerurl', 'namespace_login_headerurl' );
/**
 * Replaces the login header logo URL
 *
 * @param $url
 */
function namespace_login_headerurl( $url ) {
    $url = home_url( '/' );
    return $url;
}

add_filter( 'login_headertitle', 'namespace_login_headertitle' );
/**
 * Replaces the login header logo title
 *
 * @param $title
 */
function namespace_login_headertitle( $title ) {
    $title = get_bloginfo( 'name' );
    return $title;
}

add_action( 'login_head', 'namespace_login_style' );
/**
 * Replaces the login header logo
 */
function namespace_login_style() {
    echo '<style>.login h1 a { background-image: url( ' . get_template_directory_uri() . '/images/logo.png ) !important; }</style>';
}

EDIT: If you want to use the site logo to replace the login logo, you can use the following to dynamically pull that information (tested on WP3.5 ):

function namespace_login_style() {
    if( function_exists('get_custom_header') ){
        $width = get_custom_header()->width;
        $height = get_custom_header()->height;
    } else {
        $width = HEADER_IMAGE_WIDTH;
        $height = HEADER_IMAGE_HEIGHT;
    }
    echo '<style>'.PHP_EOL;
    echo '.login h1 a {'.PHP_EOL; 
    echo '  background-image: url( '; header_image(); echo ' ) !important; '.PHP_EOL;
    echo '  width: '.$width.'px !important;'.PHP_EOL;
    echo '  height: '.$height.'px !important;'.PHP_EOL;
    echo '  background-size: '.$width.'px '.$height.'px !important;'.PHP_EOL;
    echo '}'.PHP_EOL;
    echo '</style>'.PHP_EOL;
}

Enable Hidden Administration Feature displaying All Site Settings

Tested on: WordPress 3.1 RC3

This little piece of code does something pretty cool. It will add an additional option to your settings menu with a link to "all settings" which will show you a complete list of all the settings you have within your database related to your WordPress site . The code below will only made this link visible to an administrator user and hide it for all other users.

// CUSTOM ADMIN MENU LINK FOR ALL SETTINGS
   function all_settings_link() {
    add_options_page(__('All Settings'), __('All Settings'), 'administrator', 'options.php');
   }
   add_action('admin_menu', 'all_settings_link');

Include custom post types in the search results.

// MAKE CUSTOM POST TYPES SEARCHABLE
function searchAll( $query ) {
 if ( $query->is_search ) { $query->set( 'post_type', array( 'site', 'plugin', 'theme', 'person' )); } 
 return $query;
}
add_filter( 'the_search_query', 'searchAll' );

Add your custom post types to your sites main RSS feed by default.

// ADD CUSTOM POST TYPES TO THE DEFAULT RSS FEED
function custom_feed_request( $vars ) {
 if (isset($vars['feed']) && !isset($vars['post_type']))
  $vars['post_type'] = array( 'post', 'site', 'plugin', 'theme', 'person' );
 return $vars;
}
add_filter( 'request', 'custom_feed_request' );

Include custom post types in "Right Now" admin dashboard widget

This will include your custom post types and the post counts for each type in the "Right Now" dashboard widget.

// ADD CUSTOM POST TYPES TO THE 'RIGHT NOW' DASHBOARD WIDGET
function wph_right_now_content_table_end() {
 $args = array(
  'public' => true ,
  '_builtin' => false
 );
 $output = 'object';
 $operator = 'and';
 $post_types = get_post_types( $args , $output , $operator );
 foreach( $post_types as $post_type ) {
  $num_posts = wp_count_posts( $post_type->name );
  $num = number_format_i18n( $num_posts->publish );
  $text = _n( $post_type->labels->singular_name, $post_type->labels->name , intval( $num_posts->publish ) );
  if ( current_user_can( 'edit_posts' ) ) {
   $num = "<a href='edit.php?post_type=$post_type->name'>$num</a>";
   $text = "<a href='edit.php?post_type=$post_type->name'>$text</a>";
  }
  echo '<tr><td class="first num b b-' . $post_type->name . '">' . $num . '</td>';
  echo '<td class="text t ' . $post_type->name . '">' . $text . '</td></tr>';
 }
 $taxonomies = get_taxonomies( $args , $output , $operator ); 
 foreach( $taxonomies as $taxonomy ) {
  $num_terms  = wp_count_terms( $taxonomy->name );
  $num = number_format_i18n( $num_terms );
  $text = _n( $taxonomy->labels->singular_name, $taxonomy->labels->name , intval( $num_terms ));
  if ( current_user_can( 'manage_categories' ) ) {
   $num = "<a href='edit-tags.php?taxonomy=$taxonomy->name'>$num</a>";
   $text = "<a href='edit-tags.php?taxonomy=$taxonomy->name'>$text</a>";
  }
  echo '<tr><td class="first b b-' . $taxonomy->name . '">' . $num . '</td>';
  echo '<td class="t ' . $taxonomy->name . '">' . $text . '</td></tr>';
 }
}
add_action( 'right_now_content_table_end' , 'wph_right_now_content_table_end' );