Google Best Kept Secret Service?

A great thread at DigitalPoint Forums asks member to post what is "Google best unknown feature." There aren't too many posts yet, but pretty much all the answers are unique.

Answers include:

• Google Alerts
• Google Spreadsheets
• GMail
• GTalk
• Googles Webmaster tools
• Google Analytics

So what is your favorite Google service?

Forum discussion at DigitalPoint Forums.