The 7th and last pre-Republic King of Rome; full name Tarquinius Superbus, Lucius, but known in history as Tarquin the Proud (?-496 BC). Superbus doesn't REALLY mean proud -- it's a pejorative term and something like "haughty" or "arrogant" is more fitting.
He reigned 534-510 BC, and he was apparently the son of Tarquinius Priscus and the son-in-law of the sixth Roman king, Servius Tullius. An autocrat by nature, he tried to reduce the powers of the ordinary Roman (or rather, the ordinary rich Roman) much to their chagrin. He is said to have purchased three prophetic books from the Cumaean Sibyl (who lived on the West coast near Baea), and to have deposited them in the temple of Jupiter begun by his father.
After his son raped the Roman matron Lucretia -- the wife of Lucius Junius Brutus -- and Lucretia then committed suicide, a popular uprising dethroned Tarquinius and expelled his family from Rome. Brutus, who was actually the nephew of Tarquinius, led the revolt and proclaimed the Roman Republic in 509 BC.
Some scholars have suggested that Tarquinius Superbus and Tarquinius Priscus may have been the same person, and that later tradition was embellished by stories of Greek origin to distinguish two kings, but no compelling evidence has been presented for doubting the existence of both men.
(some thanks due to Encarta!)