Today on virtually all Google home pages is a special Google Doodle to commemorate the last day of the Mayan calendar, also known by many as the "end of the world." But that Doodle / Google logo is not on Google.com (USA), instead you have the standard Google logo with a green ribbon under the search box to remember those lost in the terrible tragedy a week ago today in Newtown, Connecticut.
Here is a picture of the logo and ribbon in the US:
As you can see, when you hover your mouse over the green ribbon it says "Remembering those lost at Sandy Hook Elementary School."
This all happened just about a week ago (in 1.5 hours from now) about an hour away from where I am now. I feel so terrible for all those who lost loved ones.
Google often will not post a scheduled Google Doodle when there is sorrow in that area.
Now, if you go to other Google home pages, like Google UK, you will see the Mayan logo. Here is a picture:
A lot when into this logo. It was developed and designed by Miguel Alva who has a post written in Spanish explaining it all at this Google blog.
In short, the world is not ending. This calendar cycle ends and repeats itself. The logo itself represents the 13th Baktun, forming the word Google. Here is a diagram Miguel put together explaining it, if you understand Spanish.
Update: English version - click to enlarge:
Forum discussion at Google+.