Over 100 years ago, a sketch was made of a potential $100 face value coin (free WSJ Yahoo! link). The coin was never made, but the sketch's discovery makes for good cocktail party chatter among numismatists. The details:
Such a coin would strictly have been used for commerce between countries, rather than general circulation, due to its hefty weight.
The sketch in question is one of many drawings contained in the so-called George T. Morgan sketch book. Mr. Morgan created one of the most popular silver coins, the Morgan silver dollar. That is the coin that appears in many Hollywood Westerns.
You may be asking yourself, "what is the highest face value U.S. coin actually made". It's the American Platinum Eagle. The market value of these coins is much higher than the face value though, so don't go using them as legal tender, even though you can.