So far, I've unlocked one of the secret hats. I'm wondering what other secret hats are out there and how to earn them?
Now that Winter Bash 2014 is over, a wrap-up blog post has been written: So long Winter Bash 2014
Chameleon was supposed to be awarded to users when they change their avatar, but a bug triggered it in other situations, such as editing other profile fields or creating a linked account.
Eureka! was manually awarded by the Stack Exchange staff to users who independently guessed how to get any secret hat (except Eureka! itself, which first appeared during Winter Bash 2013).
Fascinating was earned by upvoting an accepted answer that had a score of at least 5.
Fascinating, Ma'am was earned by upvoting a question that had a score of at least 25 and 10K views.
Hairboat was earned by commenting on one of abby hairboat's posts or replying to one of her comments.
Time Lord was earned by editing five questions that were posted more than a year ago.
Waffles was earned by downvoting a question, editing it (approved suggested edits count) and then changing your downvote to an upvote.
Warm Welcome was earned by upvoting any user's first post.
Please note that I (The Guy with The Elf Hat) did not figure out any of these hats by myself; I merely compiled all the information into one answer.