Steph Davidson: http://paralleluniver.se/

I recall this being at the top of /r/webdev 3 or 4 years ago. It turns out that a lot of the subsections have their own themed 404s as well.

My personal favorite: https://www.bloomberg.com/politics/404

Bloomberg are normally described as a reliably joyless organisation, so it's nice that employees are allowed to have SOME fun.

In a similar spirit, the Bloomberg Businessweek magazine used to have zany cover designs (my favorite featured a sweaty Steve Ballmer in a Clippers jersey dribbling a basketball captioned "BASKET BALLMER") but they've since switched to plain, solid colors.