ooheadsooIt's depends how big your key head is. I have some square ones and they work great one handed where you can just push it out. On the other side I have my bike key which is rounder and requries two hands sometimes. Here is a pic:
ooheadsooMost standard house keys will flip out one handed no problem. Push on the backside with your pointer finger, then swing out like a knife with your thumb. :)