I adore my extra server headers. Adds a few bytes, whateva.

  % curl -I http://static.whalesalad.com/north_korea/
  HTTP/1.1 200 OK
  Server: nginx/0.6.32
  Date: Fri, 28 Jan 2011 23:44:45 GMT
  ...
  Content-Type: text/html
  X-Fueled-By: Plankton
  X-Special-Sauce: Django
  Accept-Ranges: bytes

I always liked:

X-The-Number: 09-F9-11-02-9D-74-E3-5B-D8-41-56-C5-63-56-88-C0