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:
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+.