Health care law
Can congress force U.S. citizens to buy health insurance?
But it can tax, and it can choose the means and circumstances under which it taxes.
The constitution forbids imprisonment on basis of unpaid debts. Thus there is no "real" force is being leveraged, because the person's freedom is not threatened by their failure to pay the fine.
The health care mandate is a tax that is leveraged on all people who do not have health insurance. If you don't pay it, you may face fines, but no jail time. (your wages will eventually be garnished, though)