On a more casual level, there are a variety of service businesses that make great side businesses. Sites like Craigslist can be a great way to put your skills to use and piece together revenue. Do you know what interviewers look for? Consider offering resume building help. Know multiple languages? Put your skills to good use and offer tutoring or language conversation lessons. Do you love animals? Advertise yourself as a dog walker for your neighborhood.
I surprisingly get a lot of people asking to detail their car. I never intend to make a business of it, but I love doing it to my cars and people ask me to do it to theirs. All it takes is a cheap orbital buffer (mines a used craftsman) and a shop vac. I normally get easily $100 for a basic wash/wax/vac, or $200 to remove scratches and polish then wax the car.