Just logged on to the site, and I noticed a curious new link at the top of the page. What does it mean?
It means, it's time for HATS!
The Winter Bash is a promotion, where doing common tasks throughout the community can earn you special gifts. It you click the Winter Bash link at the top of the page.
A popup window will open, which lets you view all the hats you've earned.
From here you can click the Winter Bash link, which will take you to a page where you can learn more about how to earn hats.
If you don't like, don't want to look at, or just plain can't stomach hats. Clicking the i hate hats link will make known your hatred of hats, and you will not have to see them ever again (at least not on a StackExchange site).
So get out there asking, answering, voting on, and reviewing questions. See how many hats you can earn.
To Wear A Hat
Once you've earned a hat, or 5. You can wear the hat proudly upon your Gravatar, simply by selecting it from your user profile page.
During Winter Bash, you'll notice a new item in the stats area of your user profile labeled Hats.
Once you have a hat, click the link and a dialog will open.
Simply select the hat you'd like to wear, and close the dialog. You'll notice (usually after a refresh), that your Gravatar is now sporting the new hat.
Good luck, and may the hats be with you.
For more information, check out the official StackExchange Blog article.