CodeCanyon - Next Post Fly Box v2.1.1 For WordPress | 811 KB
Next Post Fly Box For WordPress is a WordPress plugin that creates navigation between next and previous post as a floating pop-up box on the left or right side of the window. When the user scroll the page a fly box is showing, allowing the user to select next or previous post based on conditions set in the plugin options panel.
DEMO:
http://codecanyon.net/item/next-post-fly-box-for-wordpress/3020948