Pedal point
When my youngest daughter was born in 1988, I realized that my days of maintenance-free health were over. I was twenty-seven years old and starting to thicken around the middle. I enjoyed running recreationally in college, but hadn't done much exercise at all for the few years since graduation.

The hook
I went looking for a form of exercise that I'd stay interested in. I didn't want a piece of exercise equipment that would be primarily used for hanging laundry. Mountain bikes had really caught on at the that time, so I looked into them. I was pleasantly surprised by the sturdiness of the frame, the width of the tires, and the strength of the brakes.
I spent $350 on my first adult bike, a Schwinn Impact, a chromium-molybdenum frame with Sun Tour componentry. I was pleasantly surprised to find that a high-quality bike suitable for commuting was available for my youngest daughter for around $400 in 2005.

Accessorize
Bicycle shops make very little on the sale of a bicycle. They make their money from repair, maintenance, and sale of accessories. Most bike shops will offer 10% off accessories at the time of sale of your bike. At the very least, a good commuter bike should have a helmet, rear rack, fenders, front and rear lighting, and either panniers or fine grid baskets.
I was very frugal and chose folding wire baskets that fastened to the rear rack. When not in use, they folded and clipped out of the way. It was not until I had been a faithful commuter for six years that my wife bought me a nice pair of roomy panniers. The answer to "What bike (or accessories) should I buy?" is the same as what the reputable computer salesman tells you: It depends on what you want to do with it.

Length of the commute
When I bought that first bike, my apartment was less than a mile from the school where I taught. After teaching summer school, my job remained the same, but was moved to a different school, about three miles away. The following year, we moved to an apartment in the suburbs, lengthening my commute to five miles. A few years later, I became a junior high librarian and my commute extended from my home in the southern suburb to my new school in the northern suburb, eleven miles each way for a weekly total of 110 miles.

Through rain, sleet, snow, dark of night...
I was determined to make my investment really work for me even through the cold, often deep in snow northern Utah winters. One of my favorite commuting memories is listening to the school secretary telling a teacher on the phone that "Yes, I know the weather's terrible, but school has NOT been cancelled and sorry, you need to come in." She looked up and saw me brush half a foot of snow of my rear rack before entering the building.
A rule of thumb that has served me well is: If I don't feel well enough to ride my bike to work, I don't feel well enough to work at all.
It became a real point of pride when people would ask, "Did you ride TODAY?" Those people really inspired me. My hope is that I've inspired a few of them too.

Folks ask me often WHY I ride to work every day. Frugality? Environment? Exercise? Recreation? For me, it started off as exercise, but has spread to all of these and more.

So, let me know how and why you commute by bicycle. What do you ride? How have you outfitted it?
Let me know why you DON'T commute by bicycle. What would it take to get you on a bike even once a week?