Darth Maul's Lightsaber
Darth Maul wielded a double-bladed lightsaber in the years leading up to the Clone Wars. With it he killed Qui-Gon Jinn and duelled Obi-Wan Kenobi, but the young Jedi cut his sabre in half and mortally wounded Maul. When Maul resurfaced during the Clone Wars he wielded the same sabre, albeit with only one blade.
Satele Shan's Lightsaber
Satele Shan wielded a double-bladed lightsaber during the Old Republic era. She duelled Darth Malgus and his master on a ship in orbit above Korriban and escaped them, but went on to fight Malgus again on Alderaan. She defeated the Sith, but only after he cut her sabre in two.
Pong Krell's Lightsaber
General Pong Krell wielded two double-bladed lightsabers, one blue and one green. With them he killed several Clone Troopers of the 501st Legion until he was taken down by the clones he betrayed and executed.