Update May 2009: The growing popularity of farmer's markets offers another venue for selling your goods. Make sure to check the health department rules for your area, though.
In my area of the country, I often see the Amish set up along busy highways selling freshly-baked goods. They do a healthy business. Fellow blogger Amey Stone shared with me an excellent blog experiment on the potential for selling bottled water at a freeway exit, in which a blogger sold a case's worth of water in less than 30 minutes. In the days when I ran a very large bicycle tour, we had people who followed our tour for the week, selling iced-down soda and other treats along the route, and pulled in some very generous profits. A friend who is in the wholesale flower business makes a point of disposing of too-old bouquets in a way that avoids further damage, knowing that people will pick up those discards and sell them at roadside.
Roadside sales can pull in some quick cash. Pick high-profit items that aren't highly perishable and/or have a high cost/sale price ratio; cold soda, corn, baked goods, flowers. Pick a location that is safe, where passersby can park without hazard, a location that has a good traffic flow, and one for which you have permission, or at least is not illegal. (Freeway exits are not usually legal sales areas.) You may be required to have a vendor's license, also.
Don't expect to get rich, or minimize the work involved. But if you really need some quick cash, roadside sales can help you over the hump. And it will leave you with more pride than those simply begging.