Never underestimate the power of a kind word or deed.

Never underestimate the power of humor.

Never underestimate yourself.

Never underestimate the impact you may have on someones life.

You can never underestimate the stupidity of the general public. - Scott Adams

"Two Things are infinite. The universe and human stupidity.
...And I'm not sure about the universe" - Einstein
Did Einstein really say that? – Quote Investigator - Maybe.

"Never underestimate the power of stupid people in large groups." - George Carlin
I couldn't find where he actually said that, but it's attributed to him in many places.
See The American Dream.

"Never underestimate the power of greed"