It was the Quakers who came up with the concept of solitary confinement. As Brooke Shelby Biggs, the author of a fine Mother Jones article on the subject tells it, when the Quakers built their Walnut Street jail in Philadelphia in 1791, it was revolutionary, the first prison designed to not only house inmates as they awaited execution, but possibly to rehabilitate them as well so that they could return to society once more. In return for that offer of earned forgiveness from the Quakers, prisoners were kept in solitary confinement, in part so they would have ample opportunity to reflect on their crimes, contemplate how they could be better people, perhaps even chew their fingertips off. That last one was an indirect and unintentional result of solitary confinement: It had the tendency to drive some people insane.
The great Quaker experiment of solitary confinement perhaps largely failed at rehabilitating prisoners, but it did show it's an excellent way to punish and keep a thumb on obstinate and rowdy prisoners, which is how it's still used today in modern penal systems across the world.
I was reminded of the Quaker experiment by another experiment in prisonry I recently came across, this one in Norway. Off the coast of Oslofjord is a tiny island called Bastøy, an island so tiny, one can traverse its entire circumference by foot in around an hour. This island is a prison island, like Alcatraz in San Francisco, Port Arthur in Tasmania (that's nearly an island), or Robben Island in South Africa. But unlike its ilk, Bastøy doesn't have cells or armed guards. Instead, it more resembles Australia when it was a continent-wide penal colony for Great Britain. Prisoners are left there and allowed to make do on their own.
It is true there are guards there, but they are unarmed. There are no guns present on the island. There is a supermarket and newly arrived prisoners are given 500 kroner (about $92) when they reach Bastøy. The doors to the buildings that house the inmates aren't kept locked. The 115 men are free to come and go as they please, so long as they remain on the island.
And it's not impossible to get off the island. The distance between Bastøy and the mainland is swimable, there's a ferry that comes and goes at least once a day, there are boats and the channel even freezes over enough to walk to shore in the winter. Instead the convicts brought here -- muderers, rapists and other thugs among them -- are enticed to stay through the offer of potential rehabilitation by using Bastøy as a place to relearn (or learn) how to make one's own way honestly in the world. Succeed or fail, it still shows the humanity in humankind that we're still willing to experiment with ways to forgive.
Here's the entire Der Spiegel article on Bastøy (thanks, LOML).