There is a lot written on this subject. Please see this link for a capsule; also, the recent film Religulous touched on some of this.

http://www.freethoughtpedia.com/wiki/Was_the_bible_plagiarized_from_other_works_of_fiction%3F

The highlights are paraphrased below (the latter parts are directly quoted):

In particular, the Bible's "flood story" in Genesis closely resembles The Epic of Gilgamesh.

Also, Jesus similarity to many previous characters, messiahs, or gods is well documented, including the following:

* Asklepios (healed sick, raised dead, the "redeemer")
* Hercules (born of a God father and mortal mother, savior of the world. His birth was foretold by prophets who said he would be a king; a leader searched for him trying to kill him. He walked on water, died and went to heaven)
* Dionysus (son of God, born of a virgin mother, rode a donkey, healed sick, turned water into wine; killed, resurrected, immortalized, and died for the people)
* Osiris (born of a virgin, king of the people, died, rose from the grave and went to heaven.)
* Osiris's son, Horus ("light of the world", "The good shepherd", and "the lamb," "The way, the truth, and the life." His symbol was a cross.)
* Mithra (birthday on the 25th of December, birth witnessed by shepherds who brought him gifts, 12 disciples, had final meal before going to heaven. On judgment day he'll return to pass judgment on the living and the dead. The good will go to heaven, and the evil will die in a giant fire. His holiday is on Sunday (he's the Sun God). His followers called themselves "brothers" and their leaders "fathers". They had baptism and a meal ritual where symbolic flesh and blood were eaten. Heaven was in the sky, and hell was below with demons and sinners.)
* Krishna (miraculous conception, wise men were guided by a star, ruler tried to find him and have him killed, parents were warned by a divine messenger and escaped, met by shepherds. The boy grew up to be the mediator between God and man.)
*Buddha (mother was told by an angel she'd give birth to a holy child destined to be a savior; teaches the priests as a child in his temple about religion while his parents look for him. He starts his religious career at roughly 30 years of age and is said to have spoken to 12 disciples on his deathbed. One of the disciples is his favorite, and another is a traitor. He and his disciples abstain from wealth and travel around speaking in parables and metaphors. He called himself "the son of man" and was referred to as, "prophet", "master", and "Lord". He healed the sick, cured the blind and deaf, and he walked on water. One of his disciples tried to walk on water as well but sunk because his faith wasn't strong enough.)
* Apollonius of Tyana (a contemporary of Jesus) (performed countless miracles (healing sick and crippled, restored sight, casted out demons, etc.) His birth was of a virgin, foretold by an angel. He knew scripture really well as a child. He was crucified, rose from the dead and appeared to his disciples to prove his power before going to heaven to sit at the right hand of the father. He was known as, "The Son of God".