Last reference to a fight on the floor in the federal government is Feb 22, 1902. Sen. Tillman (D-SC) attacked Sen. McLaurin (D-SC) over a disagreement regarding annexing the Philippine Islands. Sen. McLaurin supported annexation, a Republican proposition, which Sen. Tillman called "treachery". In response, the Senate censured both men.

At the state level, a fight happened as recently as 2011 between two Californian assemblymen.

http://www.senate.gov/artandhistory/history/minute/Senate_Fistfight.htm