In our forum, a poster asks what might produce might be good to grow and sell at the farmer's market or other venues. I threw out a few suggestions below, but please share what successes (and failures!) you've had with growing veggies, fruits or herbs for the market.
In growing produce for the market, some of the best choices will depend on the type of soil you have, your climate, and what's already available in your local area.
That said, I recommend checking out the local farmer's market and talking to farmers as a good first step. See what they're growing, what they've had success with, and what produce seems to be missing or lacking in variety in the farmer's market selection.
Two things I can think of off the top of my head that are good almost anywhere are mesclun greens and berries. Mesclun greens are easy to grow and harvest and quick to set up, but are really early season veggies. Berries are a bit more of a time commitment, both in growing and in startup time (because it takes a couple of seasons for them to start producing). But selling them at the farmer's market and even perhaps expanding to a small you-pick farm can be good income-producing options.