I agree.
So do I.
Me too.

Me neither.
 (Agreeing about a negative idea.)
I don't either.
(Agreeing about a negative idea.)
 
You're right.
That's right.
Good idea.
I think that's a good idea.

Disagreeing

I disagree.

I don't think so.

(No.) That's not right.

Yes, but...

(I'm sorry, but) I don't agree.