Jews consider that Jesus was a only a teacher and leader, assuming he existed. Some of them think he only created a Jewish sect among many other Jewish sects, and that the one who really founded what we call Christianism was Paul ^[1]. Jesus was not the Messiah because he did not fulfill the mission of the Messiah. For example, he was not a real king, he did not bring peace to the world and he did not come from the lineage of David (since Joseph was not his real father). Therefore, Jews are still waiting for the Messiah.

Early Christians had very divergent opinions about Jesus Christ before the counciles unified the doctrine. During the first decades, some Christians thought that Jesus was about to come back very soon. Some denied that Jesus had been crucified and asserted that Simon the Cirenean (Basilidians) or Judas Iscariot (Gospel of Barnabas) had taken his place. Docetists said that Jesus was only a God and not a man; his birth, Passion and death were an illusion.

Arius thought that Jesus was a creation of God, i.e he was not to be put on the same level as the Father. His doctrine was condemned by the Nicene council, but was very successful during the 4th century. Arianism may have become the mainstream flavour of Christianism.

Catholics think that Jesus was the son of God, and a part of the Holy Trinity (the other two being the Father and the Holy Spirit, in no particular order). He was born of a virgin, made miracles, died and resurrected. During Mass, the substance of bread and wine is replaced by the substance of his real flesh and blood (transubstantiation). He saved us from Adam and Eve's sin. He will come sooner or later. And yes, Jesus loves you.

Protestants think that Jesus is the incarnation of God, and is also a part of the Trinity. Luther thought that the flesh and blood coexisted with the substance of bread and wine in the Mass (consubstantiation). And yes, Jesus still loves you.

Muslims believe that Jesus (also known as Isa'l-Masih, Isa bin Maryam or simply Isa) was born from a virgin named Mary by the power of Allah. He was one of the great prophets of Allah, like Moses, and the last one before Muhammad. He was not the son of Allah, but he was Allah's Messiah, and he will eventually return. He also made miracles such as curing people or raising them from the dead. Jesus was not really crucified; the Jews crucified an image of him, or maybe someone else: the Jews plotted and Allah plotted: but of those who plot, Allah is the best.

Mormons believe that Jesus was the Son of God, the promised Messiah, only begotten. Jesus himself in his resurrected form created the Church of Jesus Christ of Latter Day Saints and leads it today (thanks xWakawaka). The Journal of Discourses says he was married at Cana ^[2].

Archaelogists and historians know almost nothing about him, since the Gospels' purpose was spiritual, not historical, and the name of Jesus is very rarely mentioned outside of them. We have probably no writing by anyone who actually saw Jesus.

The Monty Python think that Brian might very well have been Jesus, if things had only turned out that way.

I think that the gospels would be much more funny if Brian had been Jesus.

Buddhists, shintoists and ancient Greeks have no opinion about Jesus Christ. However, you should read quijote's interesting writeup about Hindus below.

Atheists think Jesus was not a god at all. Some of them doubt he ever existed.

Agnostics have no opinion: Jesus may be God, or he may be a normal man, or he may be an alien, or he may be an avatar of Elvis. Nobody has ever proved that Jesus was not an avatar of Elvis, and agnostics are too wise to reject any opinion unless it has been rationally refuted.

The greatest mystery of all is what Jesus considered himself to be. Most of the time he speaks like a preacher and a reformer, and he compares himself to a prophet (Matthew 15, 57). In Matthew 16, he says he's the Christ, i.e the Messiah, which usually meant a king (not a God) sent by God. However, nobody knows what Jesus really said and what his followers understood later, since he never speaks very clearly in the Synoptics.

[1] See http://www.geocities.com/SiliconValley/Campus/8065/jewsforj.htm.
[2] See http://www.concordance.com/jrd1.htm and http://personal.nbnet.nb.ca/nbbi/morm3trc.html.

About the gospels (and why none of them was directly written by apostles): http://www.geocities.com/Athens/Delphi/4027/gospelwriters1.html
About Jesus in the Quran: http://www.ualberta.ca/~sganam/Jesus.html

And many thanks to the many contributors from Everything2 who sent me some information.