I do my immediate neighbors (2) for free. People further away get charged $50. hr. (1/2 hr would be $25.) I have insurance because I've been there and done that before and learned a lesson.
I still work a regular job during the day so hours I plow are limited to usually after work. Neighbors can be hard to find when it comes to getting paid. Sometimes you have to wait a few weeks to get paid. I had one neighbor I had to wait almost two months to catch up with him. He was supposed to pay each time, In the meantime I was plowing him all winter at my cost. He's a good neighbor though so I didn't mind because I knew I'd get paid.
I'm not saying all neighbors are like this but in today's economy with so many people out of work and money is tight you'd be surprised how many will try to get something for nothing.
Like Gale said if you charge a fee its best to have insurance. If not be prepared when the neighbor calls you up in the spring to replace his torn up lawn and or driveway. If you charge a fee, have insurance and a properly worded contract of when you come, at how many inches of snow, assume no liability for property damage, charge per visit or by season, etc.