Ultramarines: Smurfs or easy prey

Blood Angels: Twilight.army
Space Wolves: Puppies
Eldar: Pansies
Orks: Not much really. Don't call them anything.
Dark Eldar: Since i'm the only one around who plays them I don't call them much. Well, that's not true. I frequently refer to them as Porn Elves.
Chaos Daemons: Once again, only guy who plays them. Don't call them anything really.
Chaos Marines: Unorganised sailors. Get it? Well, don't worry, it aint that funny anyway.
Witchhunters: Space nuns. With a serious case of PMS!
Daemonhunters: Losers

Tyranids: I usually just say something along the lines of "Bugs I see. Lieutenant, fetch me my pesticide!"
Various marine chapters: Usually sailors. And yet, not really. Because it's not that funny.
Tau: Space communists or quarterpounders. They have hooves for gods sake! They are cattle. Stop playing them, start eating them!
Necrons: I usually just make bad Terminator references and ask how my opponent feels that they are now the guvenor of california. Then proceed to call them govenators.
Guard: Imps, cowards, weaklings, inferiors, meatshields or some other reference to their uselessness. Or Nazis! Definitely Nazis! But not the jew burning kind, oh no! The lederhosen-wearing, sausage-loving, 99 luftballons-singing, nationalistic freaks kinda nazis.
Have I left someone out?