California
The Golden State
Nor Cal (when referring to Northern)
So Cal (when referring to Southern)
Southland (when referring to Southern. I really like this one, but it seems to have gone into disfavor for 'So Cal')
Unacceptable:
Cali
Frisco
The OC (as opposed to just OC)
The Sunshine State (this is Florida, not California)
Probably others, but that's a start.