Here's a very good reference worthy of being bookmarked.

As you very well know, google has many search engine servers or popularly known as Data Centers. Each data center has its own copy of google's index and during updates, you will notice some differences between these data centers.

There's a nice tool at mcdar.net if you want to check out multiple data centers side by side.

http://www.mcdar.net/dance/index.php

But the list below is more complete. It has about 519 different data centers.

http://www.seo-scoop.com/2006/06/23/current-known-google-datacenter-ip-list/