Kudos to Cristian Mezei's SEOpedia blog for a great list of secretive, hidden or long-forgotten pages behind Google's domain. There's more than 25 unique pages listed, but a few of my favorites include:

Google's Search Data Surrounding 9/11/01
Google's Search Data from September 11, 2001

It's like an early peek into Google Trends.
And

Google's AdWords Flash Demo
Google's AdWords Flash Demo

Just look at the low quality Flash animation! I love it... Way to go Cristian, this is solid gold!