I noticed that one user has a cool pirate hat. However, there is no description of what's required to get a hat like that on the winter bash page. Are there "secret" hats, or is this one given out on other SE sites?
If you know the name of the hat and want a more detailed description, you can go to http://winterba.sh and add the name in the URL, hyphenated and in lowercase.
For the pirate hat you're asking about: http://winterba.sh/shiver-me-timbers
Of course, this won't work if you don't know the hat's name. :)