Sam-
For him: Moose, Sammy, Samsquatch, and Rapunzel or Herpixian (please tell me you get the reference) when you want to mess with him.
   
For you: Munchkin, Shortround, and Little Luci (You sometimes remind him of Lucifer. He's still confused on if it's a good thing or a bad thing)

Dean-
For him: Squirrel,(usually accompanied by many squirrel and nut puns. They can sometimes get a bit nutty.) Casanova, old drunk (to mess with him), and finally hunter/my big strong hunter/my little hunter.

For you: Doll, Sweetums (he says it to annoy you), and sometimes Sweetheart.

Castiel-
For him: Pizza man, Honeybee, Cassie, Cas, and Baby blue eyes.

For you: Sweetheart, Babysitter, Angel, Kitten, and Honeybee.

Gabriel-
For him: Casanova, Gabe, Gabrielle, (to mess with him usually)and Candyman.

For you: Dolly, Sugar, Darlin', and Bunny.