Related Posts with Thumbnails for Blogger is a widget very similar to LinkWithin script but very customizable:
- maximum number of related posts - you can choose how many related posts to be displayed
- title of the widget - you can change the "Related posts" title to whatever you like
- JavaScript hosted on Google Code website so it loads very fast
- change colors and everything else by using CSS
For more info and instructions on how to add this widget to your Blogger template, see the article on BloggerPlugins.
