The whole "for free" thing is a fallacy. You can get all kinds of things back, even when you're not working for a paycheck. You get recognition, gratitude and respect. You get to learn new skills and improve yourself. You get enjoyment (if your work is fun).
What is the work that you want doing?
What is it other than money which motivates people?
Saying "please shovel all of my trash, because I'm too lazy" probably won't get many volunteers.
Saying "our homes were devastated by a hurricane, please help us clean away our rubbish - we just can't cope" will (hopefully) attract more people.
Some will do it because it feels like the right thing to do - or they may be expecting some nebulous spiritual reward. Others will work for free if they can get something tangible out of it - like work experience to help them get a job.
I think a better question is "Why do you need free work done"?
If it is for a greater good (curing a disease, helping your fellow man, fighting for something you believe in such as politics) and that person cares about those things then it may not be that hard. Look at all the campaign volunteers for the most recent election.
If you can define the work you want done as contributing to a common good--the health, broadly defined, of your community--you can find people to work for free. If the good it contributes to is primarily your own wealth, then you had better expect to pay.
