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