It has been illegal for years and as with silver, the composition and, therefore, the intrinsic value varies by date. You're asking about copper pennies, minted prior to (if I remember right) 1984. Silver is 90% silver from 1964 back.
Depends on the year. The way it currently is, it *costs* more than one cent to make a penny, but the metal content is closer to half a penny. So, don't go melting down recent pennies... or any coins cause that is illegal.
Silver dimes from before 1964 however... keep an eye out for those. Worth over a dollar I think.