You probably already know that you can donate blood, but many clinics will pay you for those donations to the tune of $30-40 each, and some will allow you to donate twice a week (but not so frequently that you become a regular face). You'll have to provide some medical history and information to qualify, but if you do, it's good cash for a good deed.P
You can also sell your hair. It may sound strange, but natural wigs and weaves all come from somewhere. Sites like Hairwork and Online Hair Affair both specialize in hair auctions. You can also list your hair on eBay. Watch out for scams (since much of the human hair trade involves poorly-paid "donors" from third world countries).P
Jokes aside, eggs and sperm will net you a tidy bit of money as well. Donated eggs can earn upwards of $1500 per donation (approx 10-15 eggs). Don't be fooled though—the process is intensive, uncomfortable, and requires several visits to a clinic to complete. You'll be screened, tested, and then the process itself takes place in an operating room. Men have it a little easier, but the returns are lower. Sperm donations can net you between $50 and $200 per donation, depending on your health, age, ethnicity, and personal background. Similarly, candidates are tested and screened, so be warned: You don't just walk into a sperm bank and offer to make a donation—most are screened out long before they're given a cup to fill, and even that donation is actually part of a months-long process.