What penny is worth half a million dollars?

None of them, but some come close.

The most valuable U.S. cents were part of the first two series struck in 1793. As of 09/2009 Numismedia shows the following retail values for different varieties in the best possible condition. Of course it's quite possible that these prices will increase over the next few years and reach that magic million.

> A variety of the 1793 "chain" design cent with the Word AMERICA abbreviated as "AMERI." is quoted at $412,000 > A second variety of the chain cent with AMERICA fully spelled out: $562,000 > Later in 1793 the reverse design was changed to a wreath. One variety has a vine worked into the wreath and is quoted at $625,000.