Sphider

Sphider is an open source search engine made in PHP:

http://www.sphider.eu/

It's easy to download and install and works with the same logic as Google: page ranking.

It helped me to understand better how Google ranks the pages and how their database looks like (of course, Google has more sophisticated techniques in their search engine that they won't reveal, but the base has been published and has been used to develop new engines like Sphider).

I recommend you to give it a try if you're involved in Web Development.