Someone around the year 1813 had the brilliant idea to arrange the cutting teeth of a saw around the circumference of a disc, prior to this, a saw invariably took the form of a long strip of material with a handle at one or both ends. This novelty, once implemented, had several effects: Most importantly it saved an incredible amount of labour by dispensing with the return stroke inherent to all linear saws. All the effort that was required to move the blade would, from now on, be spent on the act of cutting, with a small bonus of kinetic energy thrown in by the flywheel effect of the disc itself, as if to indicate that physics approves.

The other main effect that the inventor was perhaps unaware of at the time was that the gentle activity of working wood was wrenched into the machine age, and that the screaming bench saw would take its place among the accoutrements of masculine role insignia. It seems ironic that the inventor was a member of the Christian sect the Shakers who no doubt contemplated the young Jesus being taught the family business by Joseph, amidst fleece-like curls of wood shavings, his small flesh wounds pre-warning the unfortunate use of nails in later life. Such a pastoral scene would from that day on, become an initiation from father to son of overcoming the fear of using such a snarling beast, taming vicious torque and avoiding the loss of hands, or worse.

Originally the circular saw was made to replace a task that most people today greet with incredulity, that of two people spending a whole day in a pit making a single cut along the length of a bough with a two handled saw the size of a small surfboard. I have tried this, and still scoff with incredulity at the thought of it, it is exhausting, boring and quite liable to lead to physical damage if done on a daily basis, even though it is a surprisingly rhythmic, conversational activity. The Inventor had at heart the best interests of human beings under ridiculous strain.
The idea got developed a bit further, smaller saws to cut sheet material, Skilsaws that you can carry around, and as if to prove that there was a reason for effort to be required to obliterate a forest, the chainsaw.

There is a lot of fun to be had with a circular saw, my favourite is to walk up to someone, especially if he is wearing the full toolbox envy kit, admire his laser sighted, turbo plunge, lithium Ion Über-saw and then gently tell him about Tabitha Babbit, the woman that invented the first one, and that her original saw was developed from her spinning wheel.

Source (asked for by shaogo).
From the (out of copyright) book:
by Clara Endicott Sears
Published By Houton Miflin Company

Eldress Tabitha Babbitt, a clever, talented woman, and an inventor, too. The following account of her was compiled by a resident of Harvard : —

"One of the most talented women among the Shakers was Tabitha Babbitt. While watching the operations of making wrought nails, it occurred to her that they might be cut from a sheet of iron rolled to the right thickness. She told her idea to the smith; he tried it, and cut nails were the result.
One day as she was spinning, she noticed the brethren sawing wood in the old-fashioned way; she observed that one half of the motion was lost, and so conceived the idea of a circular saw. she made a tin disc, notched it round the edge, slipped it on the spindle of her wheel, tried it on a piece of shingle, found it would cut, and gave the world the buzz-saw. The first circular saw made under her instructions is on exhibition in the Geological Building at Albany, New York.
She invented the double spinning-head our grandmothers loved so well. At the time of her death Tabitha Babbitt was inventing false teeth and had already made a set in wax."

Many sources state that her original saw can be seen at Albany, New York- but I can find no trace of either the saw or the Geological Building that it was housed in, on the internet.