What is the value of a 1966 US quarter?

Answer 1

25 cents unless it's uncirculated. In that case values as of 05/2010 range from about 50¢ for the lowest-quality uncirculated coin to about $400 for an absolute top-grade coin, with an average of around $3-5 for most.

1966 quarters are anything but rare. Nearly a billion quarters were minted in 1966 as the Mint churned out oceans of the then-new clad coins to replace 1964 and earlier silver coins. Even today they continue to show up in change, though less frequently than before.

Note that US Coins dated 1965, 1966, or 1967 don't have mint marks.