Thomas Schulte

This plugin displays thumbnails for hyperlinks that exist on a WordPress page. 
It’s configurable whether you’d like to preview all, only external or specially 
marked hyperlinks.
This plugin uses the thumbshots of http://fadeout.de, a pretty cool site 
which offers the thumbnail service for free.
For the tooltip effect a library called “jQuery Tooltip” is used (currently v1.3). 
Find more about it here: http://bassistance.de/jquery-plugins/jquery-plugin-tooltip/
UPDATE 2010-08-10: 
I got a lot of responses relating to new features and I’m already planning the implementations. 
So thanks for your feedback and stay tuned for more news to arrive. 🙂

FadeOut-Thumbshots

This plugin dynamically shows a preview tooltip for hyperlinks on your WordPress site.

Francis

The aim of this plugin is to display a thumbnail when hovering over a link in the blog posts.
Thumbnails are generated by the Apercite service ( http://www.apercite.fr/en/ ). The thumbnail is generated in 20 seconds. 
You can choose the thumbnail size between 19 predefined sizes. You also have the ability to enable or disable JavaScript and Java when generating the thumbnail.
If you have a valid subsciption to the Apercite service, you can fill in you login and API key to request the update of the thumbnails. To do so, you need to tick the box in the edition interface of each post.
Feel free to contact me if you need help in finding the appropriate CSS attributes to display the thumbnails on your blog (don’t forget to mention the blog URL).

Apercite

The aim of this plugin is to display a thumbnail when hovering over a link in the blog posts.

FadeOut-Thumbshots vs Apercite