Power corrupts; absolute power corrupts absolutely

This arose as a quotation by Lord Acton in a letter to Bishop Mandell Creighton in 1887:

"Power tends to corrupt, and absolute power corrupts absolutely. Great men are almost always bad men."

William Pitt the Younger, The Earl of Chatham and British Prime Minister from 1766 to 1778, is sometimes wrongly attributed as the source. He did say something similar though, in a speech to the UK House of Lords in 1770:

"Unlimited power is apt to corrupt the minds of those who possess it"

* taken from http://www.phrases.org.uk/meanings/288200.html