Who Am I?

I am a little thing with a big meaning.
I help everybody.
I unlock doors -open hearts dispel prejudice.
I create friend- ship and good-will. I inspire respect and admiration.
Everybody loves me. I bore nobody. I violate no law.
I cost nothing.
Many have praised me no one has condemned me.
I am pleasing to those of high and low degree.
I am useful every moment of the day.
I am Courtesy.