Last year, the StackExchange Gaming site hosted an event called Hat Dash, where users earned "hats" for their gravatars by completing certain tasks (analogous to badges). Certain actions would trigger the user receiving a hat, which their gravatar could then "wear".
For example, editing a post would yield an editor-themed hat (fedora with pen) to the editor. Here are some examples from last year:
This year, StackExchange are expanding the promotion to all sites which choose to participate, and redesigning the hat selection to be more appropriate to the entire network (no severed Wampa heads). Some of it is holiday-themed, some of it is a bit silly, and all of it is (hopefully) fun!
This event will run from 19 December 2012 to 4 January 2013. Users will be able to see their entire hat collection on http://winterba.sh. That site will also have a landing page, explaining the rules and other details of the event.
Individual users who don’t want to participate, don’t want to see hats, and/or are generally anti-hat will have an "I hate hats" option available, just as the Gaming site had last year.
The only visual change to the site itself will be the presence of the hats and the "I hate hats" button in the footer.
Sites need to explicitly opt-in. I am minded to do so, unless I hear convincing objections here!