Fruit and Veg
Farmers can obviously enjoy making money at farmer's markets. Fresh produce and other farm fresh items (milk, cheese, etc) have never been more popular with consumers. Even if you just have a small vegetable garden, you'll be surprised at how much cash people will be willing to pay for organic, locally-grown produce. It can be very easy to make money with a simple stall selling nothing but fruit and vegetables.
Farmer's markets are also good options for florists to make money. Fresh flowers are popular year-round, and you can easily sell a wide variety of different flowers at your local farmer's market. A simple stall or cart can easily turn into a great moneymaking resource. Be sure to hit up several markets in a day, as if you don't sell your cut flowers at the first market they will start to wilt. Talk to stallholders to get an idea of how to start doing this, and any recommendations they have to maximize your sales so you don't end up throwing produce away.
Artists can even get in on the act. Artists can make money at farmer's markets by selling vintage prints or 'country' style items, such as decorative plates or tin cans. Other types of art can also sell very well here, as long as it's innovative and crafty, but country themed items, knitted baby clothing, and vintage items are usually the most popular options.