What were some of Karl Marx's famous quotes?

He has several, but likely the most famous is:

"Workingmen of all countries, unite." (The final sentence in "The Communist Manifesto" written with Friedrich Engels )

Additional quotes are:

"A specter is haunting Europe-the specter of communism." (The opening line in "The Communist Manifesto" written with Friedrich Engels.)

"The history of all hitherto existing society is the history of class struggles." (The first line of Section I of "The Communist Manifesto" written with Friedrich Engels.)

"Religion is the opium of the people." (In Gesamtausgabe by Marx and Engels) Note: Marx did not say "Religion is the opiate of the masses," even though they can be read as meaning the same thing. But we are talking quotes here and that is a closer translation from the original German text.

"From each according to his abilities, to each according to his needs."

(This quote is the motto of true communism from "Critique of the Gotha Programme")

"This I call the Fetishism which attaches itself to the products of labour, so soon as they are produced as commodities, and which is therefore inseparable from the production of commodities. This Fetishism of commodities has its origin, as the foregoing analysis has already shown, in the peculiar social character of the labour that produces them." - (Karl Marx, Capital Vol. 1, chapter 1 section 4)