Not only do American pennies have value as money that can be exchanged for goods and services, but they also have the intrinsic value of the metals contained within them. In most cases, actual value of the metal within the coin is far below the face value of that coin, however this is not the case of pennies minted before 1982.
Pennies today weigh 2.5 grams and are made up of 97.5% zinc and 2.5% copper and this is the way it's been since 1982. Before that time, pennies weighed 3.11g and contained a mixture of 95% copper and 5% zinc. A sharp rise in copper prices in the early 1980s caused the US Mint to change to today's ratios which makes pennies less expensive to manufacture.
Which begs one to ask the question: Has the intrinsic value of the copper in pre-1982 pennies exceeded the face value of the currency, and if so, how does that impact if the penny is worth picking up?
Utilizing the values above, we know that old pennies contain 2.95g of copper. Copper is currently trading at a price of $3,365 per tonne1. Since there are 1,000,000 grams in a tonne, we use simple algebra where x equals the number of pennies we need:
2.95x = 1000000
Gives us that we will need to extract the copper from about 338,983 pennies in order to get 1 tonne of copper.
The face value of the pennies we have is equal to $3,389.83, slightly more than the $3,365 we would recieve for the copper on the market. The face value of $.01 per individual penny is currently more than the intrinsic value of $.0099 per individual penny. However, we must realize that the price of copper only needs to rise 1% in order for the value of the copper in a pre-1982 penny to be worth more than the penny itself, and thereby changing the "value" of penny-pinching that Jurph shows above.
Thus, when one is asking the question if a penny worth picking up, one should not only take into account the cost of their food and their wages, but also the date the penny was minted and the current price of copper.
1 London Metal Exchange - May 6, 2005