Itachi's backstory:
Things were going well for Itachi at Konoha until he and his best friend got into a fight over to last stick of pocky. It errupted into a fight that killed his entire clan. During the melee, Sasuke took the stick of pocky, ate it and left without anyone noticing. Only he survived Itachi's rage. Seeing that his clan was dead and could buy him no more pocky, Itachi left Konoha to join the Akatsuki- an organization that gives him a weekly "pocky allowance" of $23.15. He is partnered with Kisame, the only guy in the group who doesn't like pocky. This is a safety precuation to prevent a repeat of the Uchiha Pocky Wars that wiped out the clan.
Kisame's backstory:
Kisame was born to the King of the Seas, King Triton. Although contact with humans was forbidden, he fell in love with a beautiful human princess, named Fuun-hime. He went to the sea witch and offered his voice in exchange for her making him human. She agreed to this deal. Unfortunately, as she was in the process of performing the exchange, they were interrupted by the arrival of a shark hunter. Kisame fled to the surface for his life. He was left half-fish and with a rather horrible voice, but he decided to go confess his love to Fuun-hime anyway. Shocked by his horrible ugliness, she fainted and went into a coma. No longer able to return to Atlantis and having been rejected by his human love, Kisame ended up joining the Akatsuki, who promised him fresh fish every weekday.
Deidara's backstory:
Deidara was raised in a Play-Doh factory in secret, since his parents were international Play-Doh smugglers and had to hide from the law. His fondest childhood memories involved making small animals out of Play-Doh and then blowing them up. His parents were eventually caught, so young Deidara made his living selling illegal exploding lawn flamingoes for redneck asassinations. He ended up joining the Akatsuki when he noticed that Itachi enjoyed watching him blow up sculptures of little animals. Having finally found someone who enjoyed his (horrible) taste in art, Deidara was very happy and vowed to improve his skills at exploding sculpture.
Sasori's backstory:
Sasori just wanted to be a real boy... but he lied so often that he became more and more puppet-like. His father, who came searching for him, was swallowed by Kisame and lost for several days before being retrieved. Due to Sasori's lying ways, his wooden nose constantly grew to great lengths, which was very troublesome for him. Then he met Deidara, an artist who said that he could use the extra wood for bracing his ceramics before firing them. Thus was born a beautiful friendship...
Zetsu's backstory:
Zetsu was born the child of a spirit-fox named Kurama and a man-eating plant that the fox decided to marry, much to the dismay and confusion of his friends. When he came of age, Zetsu's appetite for humans was no longer controlable. Grief-stricken but determined to do the right thing, Kurama sent his son out into the world so that Zetsu would eat people that they didn't know, instead of their friends and neighbors.
Tobi's backstory:
Tobi is the child of a basketball and a pumpkin. Knowing that they couldn't give their child the provision he deserved, Tobi's parents gave him up for adoption to a loving family who raised him as their own. Years later, Tobi was reunited with his birth parents, and was surprised to find out that he had three sisters. A photo of this joyous moment can be viewed here.
Hidan's backstory:
Hidan was raised by parents committed to destroying Western civilisation and instituting their own law as mandatory in all of the earth. He was very successful as a child suicide bomber, because he could blow himself up over and over without dying. He eventually got tired of this and left the Taliban in order to get a job as a lawyer, which did more to satisfy his hatred and bloodlust than terrorism did. After being sued for malpractice [i.e.- killing his clients for pleasure] eight times, he gave up law and joined the Akatsuki.
Kakuzu's backstory:
Kakuzu... no one knows where he came from because he won't sell rights to his biography for anything less than $1,000,000.00, and no one is curious enough to pay him that much.