WP Minimize Admin Bar is a simple plugin that minimizes the WP Admin Bar (the bar at the top of the page that logged in users see). Instead of the typical WP Admin Bar that remains fixed at the top of the page, this plugin replaces it with a small, unobtrusive icon in the top left corner of your site. When the icon is clicked it reveals the full WP Admin Bar that you're familiar with.

Update 2015/10/29 - Plugin now available in the Wordpress plugin directory

• Go to Plugins > Add New
• Search for minimize admin bar
• Find the correct plugin and click install

• Download a zip file of the code
• Extract files to /wp-content/plugins/wp-minimize-admin-bar
• Activate the plugin in your WordPress admin control panel

There are no plugin options at this point. You can see the plugin in action when you visit your site (the “frontend”, not the admin control panel) when you are logged in.

http://mikeeverhart.net/wordpress-plugin-minimize-wp-admin-bar/