Dog owners love their dogs and hate the backyard. A Virginia couple started scooping yards before and after their day jobs in 2000, and DoodyCalls now has more than 100 locations. At about $18 a week, 50 regular yards comes to roughly $3,900 a month.

Most people would rather see the dentist than spend a Saturday at a car dealership. A former car salesman now charges buyers a flat $1,000 to do the haggling for them, and his company brings in about $200,000 a month. He started by doing 50 deals for free.

Apartment buildings and hotels want snacks on site, and nobody at the front desk wants to sell them. One operator started with two used machines and about $4,000 and grew to more than $300,000 a year in sales. The building supplies the customers; he supplies the machine.