My friend Tim Anderson taught me this trick. You need a $30 hole punching tool, but once you've got it, any coin can be turned into a washer (and the punch is useful for other stuff too). Penny's are great because pre 1983 pennies are solid copper, and post 83 they are copper coated zinc. (I stand corrected, I originally said nickel)
Step 1: Punch Hole in Penny
Take your spare change and punch a hole in the center. Technically I think this might be illegal, but a non-corroding washer is a really useful thing. They can be quite decorative too. Use the pre-'83 ones if you want the best non-corrodability.
more info on penny history: http://ky.essortment.com/historypenny_rmor.htm
or from: http://www.usmint.gov/about_the_mint/fun_facts/index.cfm?flash=yes&action=fun_facts2
Following is a brief chronology of the metal composition of the cent coin (penny):
The composition was pure copper from 1793 to 1837.
From 1837 to 1857, the cent was made of bronze (95 percent copper, and five percent tin and zinc).
From 1857, the cent was 88 percent copper and 12 percent nickel, giving the coin a whitish appearance.
The cent was again bronze (95 percent copper, and five percent tin and zinc) from 1864 to 1962.
(Note: In 1943, the coin's composition was changed to zinc-coated steel. This change was only for the year 1943 and was due to the critical use of copper for the war effort. However, a limited number of copper pennies were minted that year. You can read more about the rare, collectible 1943 copper penny in "What's So Special about the 1943 Copper Penny.")
In 1962, the cent's tin content, which was quite small, was removed. That made the metal composition of the cent 95 percent copper and 5 percent zinc.
The alloy remained 95 percent copper and 5 percent zinc until 1982, when the composition was changed to 97.5 percent zinc and 2.5 percent copper (copper-plated zinc). Cents of both compositions appeared in that year.