Data's sacrifice in 2379, when he sent Picard to safety and destroyed the Scimitar's thalaron weapon from within, at the cost of his own existence.
Data dies at the end of Star Trek Nemesis. In the Battle of the Bassen Rift, Shinzon readies a thalaron weapon that will kill everyone on the Enterprise-E. With transporters failing, Picard beams over alone. Data then leaps across open space to the Scimitar.
He finds Picard standing over the dead Shinzon, fixes a one-person emergency transport unit to the captain and sends him home. Data then fires his phaser into the thalaron generator, and the Scimitar explodes. The crew drink to him, and B-4, who holds his memories, starts to hum Blue Skies.
Picard later shows that a copy of his mind survived in a simulation until 2399, when Data asked Picard to end it. A new android body holding Data, Lore and B-4 appears in the third season.