Trent Hamm at The Simple Dollar has compiled a great little list of 50 side business ideas that he's collected. He thinks that "each of these ideas is very simple to start, and most can be done as a sole proprietorship at first."
Here's a sampling:
Antique refurbishment This is a perfect side business for people who love antiquing. Take worn-out antiques home with you, invest the time and care needed to transform those old items into something amazing, then resell them at a profit.
Auto detailing Have a meticulous eye for detail and love to get things gleaming clean? Auto detailing is probably a perfect side business for you. In essence, your job is to make cars sparkle inside and out - and many people are quite happy to pay well for this service.
Babysitting Got lots of evenings free? Like kids? Babysitting may be a great side business for you. Keep an eye on multiple children on Friday and Saturday nights and you can earn some solid income.
Bed and breakfast Have some extra space in your home? Turn that extra bedroom into a “bed and breakfast” room. This works particularly well if you have a somewhat older home or live near an area that attracts regular travelers and tourists.
Blogging If you enjoy writing, find a topic you’re passionate about and start a blog on the topic. All you need is a computer, some time, and some energy to consistently write.
Cake decorating Enjoy baking and have a bit of an artistic touch? Learn how to decorate cakes and make them for special events. One of my mother’s old friends does this and makes quite a bit of money on the side.
Candle making Candle making is a great little craft to learn. You can often easily sell the candles at local shops and also through websites like Etsy.com.
Candy making Homemade candies are easier to make than you think and quite popular. Package them in nice little boxes and sell them through a local gift shop.