Space Marines: shpess mehrens (spoken as slurred as possible)
Eldar: Space Elfs
Deldar: Dark Space Elfs (creative, I know)
Tyranids: Nids
CSM: Chaos Marines
Daemons: Daemons
Necrons: Negrons, New Grey Knights
Tau: Tau
Orks: Fun, Greenskins, Orks, Awrks
Grey Knights: Gee-Kay
Space Wolves: Space Wolves, Wolf Fuckers, Furfags (depends who I'm talking to)
Blood Angles: Red Space Marines
Black Templar: Black Space Marines
Imperial Guard: Eye-Gee, Men, Little Green Men