In the early '40s, the mint put out a penny made of lead (war effort). Haven't seen one in decades.

Tip: Be sure to do your hunting with at least a 6x power magnifier so you don't miss anything!
1) 1969-S Lincoln Cent with a Doubled Die Obverse
This coin is exceedingly rare. The early specimens were confiscated by the Secret Service until the U.S. Mint admitted they were genuine. Counterfeits abound, but usually have the wrong mint mark.
How to Detect: Look for clear doubling of the entire obverse except for the mint mark. If the mint mark is doubled, it is a double strike, rather than a doubled die, and not worth much. (Mint marks are punched in the dies separately).
Approximate Value: Around $35,000 or more in EF-40 or so.

2) 1970-S Small Date Lincoln Cent with a Doubled Die Obverse
As with virtually all true doubled die varieties, only one side of the coin shows doubling. If both sides exhibit doubling, the coin is probably double struck instead, and worth little.
How to Detect: The rarer Small Date variety is most easily distinguished from the common type by the weakness of LIBERTY. The Doubled Die Obverse is best demonstrated by doubling in LIB and IN GOD WE TRUST.
Approximate Value: Around $3,000 in EF-40 or so.

3) 1972 Lincoln Cent with a Doubled Die Obverse
The 1972 (no mint mark) Lincoln Cent doubled die variety shows strong doubling on all elements. The "Cherrypicker's Guide to Rare Die Varieties", which was an important source for this article, suggests using a "die marker" to help verify your finds. A die marker is a gouge or crack that identifies a particular die.
How to Detect: Clear doubling of all obverse elements; look for a tiny gouge near the edge above the D in UNITED as a die marker.
Approximate Value: About $500 in EF-40 or so.

4) 1999 Wide "AM" Reverse Lincoln Cent
This variety is known for 3 dates, 1998, 1999, and 2000, with 1999 being by far the rarest. The mint erroneously used a proof die to strike normal circulation coins. How to Detect: The AM in AMERICA on the reverse is clearly separated in the Wide variety. In the normal variety for these dates, the letters AM are very close or touching.
Approximate Value: $5 to $25 in middle grades, $75 to $600 in MS-63 or better depending on color. 1999 brings the highest prices, with 2000 being second.

5) 1995 and 1995-D Doubled Die Obverse Lincoln Cents
This doubled die variety generated a lot of mainstream interest when it was featured as a cover story in USA Today. Specimens are still being found in circulation, and lesser doubling is seen in the much rarer and more valuable 1995-D.
How to Detect: Clear doubling in LIBERTY and IN GOD WE TRUST. Note that the mint mark is also doubled on this variety, as the mint began punching mint marks into the master hub in 1990.
Approximate Value: About $20 to $75 in AU-50, more in higher grades.

-- From: About.com

Bulk Lincolns to look for:
There are a lot of other doubled dies and RPM's, but a lot of them don't carry much of a premium.

1910-S RPMs
1911-D RPMs
1917 Doubled Die Obverse
1922-D Plain & weak D
1925-S Doubled die obverse and repunched mintmarks
1927 Doubled die obverse
1927-D Repunched mintmark
1935 Doubled die obverse
1936 doubled die obverse
1941 doubled die obverse
1943- P & S doubled die obverse
1943-D RPM
1944-D D/S
1947 Doubled die obverse
1955 Doubled die obverse #1 & #2 (#2 is a lot rarer, but is less spectacular and is worth a lot less)

Source: Collector’s Universe forum, June 3, 2008

Welcome to the geeky world of coin collecting - where a penny is worth $20 because the AM in AMERICA is too close or too far or whatever. If these coins could talk they'd say "THROW ME IN THE TRASH! I'M NOT WORTH THE METAL I'M PRINTED ON!"

A couple years during the war they made them out of steel, but I can't remember which years. Coin people, help me out please.

I've found that sorting through coins is a good opportunity to talk with the kids and/or encourage them to research events related to mintage dates. Tangible items have a way of engaging their curiosity in ways that books and computers sometimes fall short.

As a young child my dad brought a huge canvas sack of pennies home with coin cards for me and my two sisters. We filled those cards almost all the way up. We would sit around listening to the radio (Paul Harvey- T.V. was evil) as we dug through them. I don't remember it as fun, but nether was it painful. Just a way to pass the evening.

Dave, that was 1943, all mints.

A roll of good ones (no rust) is worth about $10.

the 1980s was a heck of a decade for pennies

Mintages were upped sharply in 1982 when the composition was changed from 95 percent copper to 95 percent zinc due to rising copper prices. The increased mintages accomodated hoarders taking old pennies out of circulation, though the return on hoarding, taking into account meltdown and transportation costs, was small to non-existent.

The current meltdown value of pre-1982 pennies is 2.2 cents -- post-1982 is 0.58 cents. Reflecting our increasingly cashless society, mintages are falling in recent years.

There never was a lead cent - you are maybe thinking of the 1943 'steelie' mentioned above. In 1942-1945, the nickel was 35% silver, to save nickel - as these tarnish, they sort of look like lead (and they are well worth saving.) There were stories a couple years back of investors with high-speed sorters that could pull the more valuable pre-'82 cents, which may account for the relative scarcity of these.