Hehe, I loved reading everyone's submissions. Niko understands:
"You want luvies?" - He'll come in my lap and nuzzle.
"Sit" or "Butt on the floor" - Yes, he sits. I also have a 'sit' hand gesture he responds to.
"High five!" - And he'll gimme one.
"Don't even think about it."
"Up" & "Down" - With a snap or tapping my finger on a surface.
"You wanna play toys?"
"Whatcha doin'?" - He responds, but alas, I don't speak cat, even if my cat speaks human. :P
"Get outta here." - If I say it jokingly to someone else, he thinks I'm speaking to him and leaves the room. LOL
"Treats" - He'll look up, but he's not a big treat eater. He'd rather have wet food.
"Who's my handsome boy?" - He'll meow and stretch out a paw as if to say, "I am!"
"It's okay" - When a noise or something startles him.
"Careful/Easy" - If he's kneading me too harshly or happens to jump on me while in play mode.
His name, he knows, but he thinks it means food... he thinks Bubbe/Bubbilah is his /real/ name. *grins* But, as like most of you had said, I think he understands everything I say. Tone of voice has a lot to do with it.. whenever I speak to him, it's always in a soft, almost baby tone- so when my voice gets deeper and sharp, he knows I mean business. I also hiss at him if words don't work, but it's very rare I have to go that far.