Agreeing

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.