What is this?
This is a php function which generates the url to the thumbnail of a given website. This function
uses the functionality provided by Google Preview. Download the source code from here.
Here
you can play around with the function.
Example
Note: For some reason, which I am unaware of, you may need to hit refresh to see some of the thumbnails. Just to mention that I am aware of this issue and I am working on it!
Another Idea
Thumbshots.org offers a similar service, only with the exception that you are obliged to include this attribution text to your page: About Thumbshots thumbnails.