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.
Note:  Click on the images to greatly enlarge them and see details.
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

Photo courtesy of Heritage Auction GalleriesThis  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.)
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.