Plugin to add a dropdown list of users in the WordPress admin bar with a link to switch to that user (requires the User Switching plugin by http://profiles.wordpress.org/johnbillion/).

gistfile1.php

<?php
/**
Plugin Name: User Switching in Admin Bar
Plugin URI: http://markwilkinson.me
Description: Build upon the User Switching plugin (http://wordpress.org/extend/plugins/user-switching/) by John Blackbourn and adds a dropdown list of users in the WordPress admin bar with a link to switch to that user.
Author: Mark Wilkinson
Author URI: http://markwilkinson.me
Version: 1.0
*/

/* alter the wordpress admin bar */
function pxjn_user_switching_adminbar() {
	
	/* include plugin file to make this work on the front end */
	include_once( ABSPATH . 'wp-admin/includes/plugin.php' );
	
	/* check whether the user switching plugin is active */
	if( is_plugin_active( 'user-switching/user-switching.php' ) ) {
		
		global $user_switching;
		
		/* load the global admin bar variable */
		global $wp_admin_bar;
		
		/* check whether the current user is super admin */
		if( is_super_admin() ) {
		
			/* add admin bar menu for switching to a user */
			$wp_admin_bar->add_menu( array(
				'id'    => 'pxjn_switch_to_user',
				'title' => 'Switch to User',
				'href'  => '#',
			) );
			
			/* set some arguments for our user query */
			$pxjn_user_query_args = array(
				'role' => '',
				'orderby' => 'display_name'
			);
			
			/* begin build of switch user url */
			$pxjn_switch_user_url_base = wp_login_url();
			
			/* create a new user query */
			$pxjn_user_query = new WP_User_Query( $pxjn_user_query_args );
			
			/* store results from user query */
			$pxjn_users = $pxjn_user_query->get_results();
			
			/* check we have users */
			if( !empty( $pxjn_users ) ) {
				
				/* loop through each user */
				foreach( $pxjn_users as $pxjn_user ) {
					
					/* get all of this users data */
					$pxjn_user_info = get_userdata( $pxjn_user->ID );
					
					/* build query args for this user */
					$pxjn_switch_user_query_args = array(
						'action' => 'switch_to_user',
						'user_id' => $pxjn_user->ID
					);
					
					/* build menu url */
					$pxjn_full_menu_url = $user_switching->switch_to_url( $pxjn_user->ID );
					
					/* build menu id for each user */
					$pxjn_menu_id = $pxjn_user_info->first_name . $pxjn_user_info->last_name;
					
					/* add admin bar menu to create a new appraisal */
					$wp_admin_bar->add_menu( array(
						'id'    => $pxjn_menu_id,
						'parent' => 'pxjn_switch_to_user',
						'title' => $pxjn_user_info->display_name,
						'href'  => $pxjn_full_menu_url,
					) );
					
				} // 
				
			} // check we have users
			
		} // check we are super admin
		
	} // check user switching plugin is active

}
 
add_action('wp_before_admin_bar_render', 'pxjn_user_switching_adminbar', 0);

Mark, I've just updated this Gist to support some changes I made in User Switching 0.8.

https://gist.github.com/johnbillion/6087274/revisions

John