Top 10 Most Valuable US Coins Found in Pocket Change

This is taken from the About.com article written by Susan Headley.  Our own comments are in parentheses in this font.

There are a number of fairly valuable U.S. error coins and die varieties in circulation today. These coins are overlooked by people because they have small distinguishing characteristics, such as a modest doubling of the coin image, or minute differences in the size or spacing of the letters in the legends. Learn which of your pocket change coins is worth a large premium over face value, and why.

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 ("heads" side) except for the mint mark. If the mint mark is doubled, it is probably a case of strike doubling, rather than a doubled die, which isn't worth much. (Mint marks were punched in the dies separately in 1969, after the doubled die itself had already been made.)
(strike doubling is common, while a doubled die is scarce)
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 probably exhibits strike doubling instead, and is 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. 2004-D Wisconsin State Quarter With an Extra Leaf

Variety experts disagree about the cause and long-term value of this type, but I've included in the list because it is very findable in pocket change and worth hundreds of dollars right now.

How to Detect: There is some defect on the die that makes it appear as if there's an extra leaf on the lower left-hand side of the ear of corn on the reverse. The leaf is very clear. Known in two varieties, the High Leaf and the Low Leaf type.

Approximate Value: $200-$300 in MS-60 or so.

5. 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 circulated condition, $75 to $600 in MS-63 or better depending on color. 1999 brings the highest prices, with 2000 being second.

6. 1982 No Mint Mark Roosevelt Dime

At the point in time that these coins were made, the dies sent to the individual branch mints would be punched with the proper mint mark letter for that branch. This variety is believed to be caused because one or more non-punched dies were used to make coins. (The letter P was being used for Philadelphia on dimes at this time.)
How to Detect: The 1982 dime is missing a mint mark.
Approximate Value: About $30 to $50 in AU-50, more for higher grades.

7. Presidential Dollar Edge Lettering Errors

Ever since the first Presidential Dollar (the Washington Dollar issued in 2007) there have been errors associated with the lettering on the edge of these coins. In some cases it is missing entirely. In others, the edge lettering has been placed there multiple times.
How to Detect: Look at the edge. The inscription should appear fully incused all around the circumference of the coin. Missing or doubled inscriptions are rare and valuable.
Approximate Value: $50 to $3,000, depending on the President.

8. 1995 Doubled Die Obverse Lincoln Cent

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 all the time!
How to Detect: Clear doubling in LIBERTY and IN GOD WE TRUST.
Approximate Value: About $20 to $50 in Uncirculated condition.

9. Certain Uncirculated State Quarters

As the economy has worsened, people who have been hoarding rolls of State Quarters have been spending them into circulation. If you can put together whole rolls Uncirculated quarters of certain in-demand states, you can get as much as $50 per roll for them.
How to Detect: Demand changes from time-to-time based on major coin dealer promotions. Currently, look for Georgia, Connecticut, Tennessee, and Illinois. Quarters must be Uncirculated!
Approximate Value: $20 to $52 per roll for strictly Uncirculated rolls of certain states. For current updates, see the State Quarter Rolls Value Guide.

10. Silver Half Dollars

Most people think that the silver in U.S. coins ended in 1964, but this isn't true. The Half Dollar coin had silver in it until 1970. Many people spend the Half Dollars from 1965 to 1970, or sells them in rolls of halves they take to the bank, not realizing they are 40% silver.
How to Detect: If the Half Dollar is dated 1964 or earlier, it is 90% silver. Halves dated from 1965 to 1970 are 40% silver. You might also find silver Proof Half Dollars, which are 90% silver and dated to current. Silver Proof Halves have very shiny, mirror-like surfaces and there is no copper color when you view the edge.
Approximate Value: Value is based on silver spot price.