I know there’s been a few cases of people paying fines with wheelbarrows full of loose pennies.

Obligatory caution that that can backfire if the recipient insists that the debtor counts the pennies. Or if the creditor refuses the pennies entirely, which is legal in some jurisdictions. (e.g. in the UK, pennies and 2p coins are legal tender up to amounts of only 20 pence. Anything beyond that is left to the discretion of the recipient.)

In the US, pennies are legal tender and have to be accepted as payment for debts owed. This tactic usually ended up in the fine being dismissed.

Pennies only have to be accepted if there is a state law saying so. For example in California if a landlord refuses a cash payment (pennies or otherwise) then the tenant basically gets free rent that month. Businesses generally set their own rules as to what is accepted. Just like you see signs saying “No bills over $20” - 50’s and 100’s are legal tender, but they can refuse them just like a bucket of pennies can be refused.

Title 31 (Money and Finance), Subtitle IV (Money), Chapter 51 (Coins and Currency), Subchapter I (Monetary System), Section 5103 (Legal Tender) of the United States Code states:

United States coins and currency (including Federal reserve notes and circulating notes of Federal reserve banks and national banks) are legal tender for all debts, public charges, taxes, and dues. Foreign gold or silver coins are not legal tender for debts.

Since you feel like quoting things is conducive to proper discusion:

Is it legal for a business in the United States to refuse cash as a form of payment?

There is no federal statute mandating that a private business, a person, or an organization must accept currency or coins as payment for goods or services. Private businesses are free to develop their own policies on whether to accept cash unless there is a state law that says otherwise.

Section 31 U.S.C. 5103, entitled “Legal tender,” states: “United States coins and currency [including Federal Reserve notes and circulating notes of Federal Reserve Banks and national banks] are legal tender for all debts, public charges, taxes, and dues.” This statute means that all U.S. money as identified above is a valid and legal offer of payment for debts when tendered to a creditor.

www.federalreserve.gov/faqs/currency_12772.htm