Update may 2009: When Illinois governor Blagojevich was perp-walked out of office, hair lost one of its greatest champions. The market is still buying, however, and what other product that you can turn into cash requires so little work?
In a truly free market economy, you'd be worth a lot of money in organs alone -- selling a kidney or lung could solve your money woes. However, the government and our sense of propriety permit you to only cash in on plasma, hair and (maybe) breast milk.
You can legally trade plasma for cash in the U.S., the fluid in which your blood corpuscles and other tiny bits circulate through your blood system. In most larger cities, there are clinics that will pay you up to $35 or so for some of your plasma, which you can donate twice a week. The process, which involves taking blood, draining off the plasma and returning the remains to your system, takes around half an hour to an hour. To donate, you'll need to be relatively healthy and drug free.
The hirsute can also make some quick cash by selling off their hair. Hair must be at least ten inches long, and uncolored. Well tended (not over-shampooed or sun damaged) hair is worth more. Web sites such as The Hair Trader serve as a market for those looking to sell or buy hair. Sales announced on the site range from several hundred to over a thousand dollars for a generous length of tresses.
Finally, some mothers who find themselves with an overabundance of breast milk have taken to offering the extra for sale on the Internet. However, the trade, of questionable legality and fraught with health issues, remains rather clandestine and I can't recommend it, even in a pinch.