Treasury bill

A Treasury bill (T-bill) is short-term US government debt, maturing in 4 to 52 weeks, sold at a discount to its face value; the difference is your interest, which is exempt from state and local income tax.

You can buy T-bills at TreasuryDirect or through a brokerage. Compare T-bill yields with savings and CD APYs after tax: in states with an income tax, a T-bill can keep more even at a lower yield.