1. Part-time job - If you have the time this is a good option.
2. Have a garage/yard sale - A good way to earn some quick cash if your willing to set it up (signs, tables, folding or sorting the items).
3. Keep a piggy bank - Look for change and store it till your piggy bank is full. Count out the change and vuola, extra money
4. Sell used clothes to thrift stores - Not all thrift stores do this, so do some research.
5. Buy items in bulk and sell them individually to make more than you bought it for - You can do this with ice-cold water bottles on a hot day, or even snacks.
6. Sell homemade crafts - If you have a thing for quilting, crocheting, knitting, or even sowing, you could sell them in craft fairs, Etsy.com (though this requires some pennies upfront), or to friends.
7. Sell yourself - No this doesn't mean the oldest profession here, you can sell your hair, eggs, sperm, plasma, and even blood at some places.
8. Swagbucks.com - I've always been skeptical about online things, but you can earn $5 to $100 gift cards to Wal-mart, Sephora, Starbucks, Lowe's, Amazon.com, etc. Not only can you get gift cards, you can earn items ranging from jewelry. to t-shirts. to digital wallpaper, even to soap.
9. Paint house numbers on curbs - If you're willing to buy the materials (paint, stencils, advertising), this is a good way to make some quick cash.
10. Sell your old gold jewelry - If you have any, make sure it's being sold to a reputable company.
11. Rent out your extra room or garage - If you have the space.
12. Walk dogs, or pet sit - You can advertise this in Petsit.com or your local newspaper.
13. Become a direct sales rep. - You can do this for company's such as Avon, Mary Kay, or Amway.
14. Look for Craigslist ETC (odd jobs) section - Beware, don't get caught up in scams!
15. Sell your television and treadmill - You can watch television online instead, and save on electricity by going outside to run/walk.
16. Return your unwanted items - Returning your unopened, unwanted items is a small way to earn a little money.
17. Rake leaves, shovel snow, mow lawns, or sell lemonade - The old school way to earn money.
18. Wrap gifts - Speaks for itself.
19. Collect soda cans, plastic bottles, or scrap metal - Redeem these for money!
20. Advertise a business - You can do this by wrapping your car in a businesses advertisement, or stand at a busy intersection holding up the businesses sign, all for a fee of-course.