The fifty commemorative quarters, one for each state. being issued by the U.S. Treasury are proving immensely popular. So much so that there is talk of a new series to replace them when the fiftieth coin is issued in 2008.
I think the best choice for a new series would be NASA mission patches. The designs are attractive, historically important and most of them are more or less circular.
Mercury, Gemini, Apollo, STS . . . . So few missions total, over forty-odd years. Let's honor the pioneers of space exploration on our currency.