Have nuclear weapons ever been used?

Yes.

• In Hiroshima, Japan: deaths as of December 1945: 140,000 deaths
deaths calculated as of August 1996: 197,045 deaths

• In Nagasaki, Japan: deaths as of December, 1945: 74,000 deaths
 
• In addition, 2,045 nuclear tests have resulted both in deaths and illness for people living in the test area and in serious environmental damage.

• In the US alone, there are estimates of 49,000 deaths from radioactive fallout following nuclear tests at the Nevada Test Site during the 1950s and 60s.

http://rex.nci.nih.gov/massmedia/falloutQ%26A.html 

On the 2013 anniversary of the International Day against Nuclear Testing, the Minister of Foreign Affairs for the Ukraine,
Mr. Erlan Idrissov, published a comment in Foreign Policy Journal on Aug. 28, 2013 stating:

Kazakhstan initiated the UN resolution that led to the international
community marking August 29 as the day to reflect on nuclear
disarmament issues. The resolution commemorates the closure of the
Semipalatinsk Nuclear Test site on 29 August, 1991. Kazakhstan was
the first country in the world to close a nuclear test site on its
territory. Nearly 500 nuclear explosions took place at Semipalatinsk,
causing untold damage to the environment and the to the health of over
1.5 million people. The power of these explosions was equal to 2,500
atomic bombs dropped on Hiroshima. The radiation polluted an area
roughly the size of today’s Germany.

http://www.foreignpolicyjournal.com/2013/08/28/security-without-nuclear-weapons-is-not-a-utopian-dream/