I just bought some Federal Farm Credit Bank bonds with a 3.7% coupon below par. (But by the time I finished the transaction, they were worth above par.) They have a 30 year maturity, which would ordinarily make them not a wise investment with the current prospects for interest rates, except that these are callable, and in practice, GSE bonds like this almost always get called within a few years -- hence the quite high coupon for such a low-risk bond.
UPDATE: A little clarification. There are two reasons why bonds get called early: interest rates fall, and the person who borrowed money that the bond is financing can refinance, or the original reason to borrow the money goes away (someone sells the item or property that the bond financed). This tends to make the effective duration of these sort of bonds much shorter duration than the nominal term.