The Nd5 sacrifice in the Sicilian
The Nd5 sacrifice is a thematic idea in the sicilian defence. If black captures it it will open the center for white's attack. White often uses it to open lines for the attack or to break through when he has a huge lead in development.
Sometimes when black accepts it white will win by force - or at least win his piece back while maintaining the attack, other times white just gets the initiative for it with an unclear position.
It also can happen when black spends a tempo to chase off the c3 knight and retreating would leave white with an inferior position.
Here are some examples: