Umar ibn al-Khattab
Second of the four "rightly guided" caliphs. Umar was born in Mecca around 581 to the Adi clan of the Quraish tribe. Umar belonged to a family of average class, but he was able to become literate, and was well known for his physical strength, becoming a champion wrestler. When Muhammad first declared his message of Islam, Umar believed Islam was heretical rhetoric against the Quraish and his ancestors, and he resolved to kill Muhammad. He was stopped on his way to Muhammad's house, however, with news of his sister's conversion to Islam. Umar was initially angered by the news, but after reading some of the Quran he was instantly changed. Rather than killing Muhammad, he determined to accept Islam.
Umar was part of the first emigration to Medina, and became an important companion of Muhammad. He participated in all of the Muslim battles against the Quraish. Upon the death of Muhammad, Umar was in such a state of despair that he threatened to decapitate anyone who said that Muhammad was dead.
Abu Bakr became the first successor to Muhammad. During Abu Bakr's short reign as caliph, Umar was an important advisor to him, and Abu Bakr selected Umar as his successor prior to his death. Umar reigned as caliph from 633 until his assassination in 644. Umar's time as caliph saw the Islamic empire grow at an unprecedented rate, taking Iraq and parts of Iran from the Sassanids, and thereby ending that empire, and taking Egypt, Palestine, Syria, North Africa and Armenia from the Byzantines. Umar also codified Islamic law, and was known for his simple lifestyle and modest living. A famous story tells of him arriving in Jerusalem walking beside his camel upon which his servant was sitting.
Umar was murdered in 644 by a Persian slave who was angered by a personal quarrel with Umar; he stabbed the caliph six times as Umar led prayers in Masjid al Nabawi. Umar died two days later, and is buried alongside Muhammad and Abu Bakr. Prior to dying, he appointed a council of six men to elect his successor from amongst themselves, chosing Uthman ibn Affan. Umar is most recognized for originating most of the major political institutions of the Muslim state and stabilizing the rapidly expanding Arab empire.
Sources: Saudi Aramco World, (January-February 2002); Wikipedia