itgoldman

This plugin shows on hover sub menus of recent posts, pages and custom post types. 
It’s a simple no-settings out-of-the-box solution for better productivity when using the wordpress admin.

ITG Admin Hover Menus

Shows on hover sub menus of recent posts, pages and custom post types.

8MediaCentral

Ever wanted to make your content more visual? With PicTips, you can! Hovering over an item with a PicTip enabled will display your chosen image.
PicTips features the following:
<ul>
<li>Displays images as tooltip bubbles</li>
<li>Automatically resizes and positions bubbles as required</li>
<li>Customise look and feel of bubbles for your site</li>
<li>Graceful fall back for non-compatible browsers</li>
</ul>
Please raise any support queries on the support forum.
Want more styles for PicTips? Try downloading an <a href="http://8mediacentral.com/developments/plugins/pictips/" rel="nofollow ugc">Add-On Style Pack</a>

PicTips

PicTips provides a shortcode for images to be used as ToolTips. Like ToolTips but with pictures.

ITG Admin Hover Menus vs PicTips