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 . 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 .
Archaelogists and historians know almost
nothing about him, since the Gospels' purpose was spiritual, not
historical. They usually think that Jesus went preaching around Judea for a year or two, and was eventually crucified by the Roman authorities. The only source is Josephus' account , but its historical value is a highly debated topic (thanks mirv!)
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. shyHyena mentions an interesting discussion on this subject at http://www.tektonics.org/tekton_01_02_01.html.
 See http://www.concordance.com/jrd1.htm and http://personal.nbnet.nb.ca/nbbi/morm3trc.html.
 See http://www.geocities.com/SiliconValley/Campus/8065/jewsforj.htm.
 Flavius Josephus, Jewish Antiquities, book 18, chapter 63. About its historical value: http://www.earlychristianwritings.com/testimonium.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 mirv, mr100percent, Spasemunki, ymelup, proj251, WyldWynd, xWakawaka, Footprints, hotthamir and others who /msg'ed me some information.