US Banknotes

Depends on the series. If its series 1976-present it is quite common and only worth $2 unless in crisp uncirculated or if it has an unusual serial number (such as 0000000001 or 777777777, etc.). If its prior to series 1976 (if it has a seal other than green) it is worth more than $2, but the true value depends on the series. For example, a series 1953 United States Note $2 in poor condition may be only worth $2.25 while a 1899 $2 silver certificate may be worth $30+ even in less than perfect condition. In general, the older the series the more the bill is worth in the same condition, but a perfect condition 1953 $2 might be worth more than a 1928 $2 in poor condition.

The yellow seal indicates that you have a special bill printed for use by Allied troops in North Africa during WWII.

If the bill has an "A" next to the date, it's the more common variety; as of 11/2012 its retail value is $20 to $60 in average condition and $200 or more if uncirculated.

If there's no letter next to the date you should have it examined by a currency expert to make sure it's genuine. These early bills are quite scarce and sell for $1200 to over $10,000.

The 2 dollar bill is in circulation, but it is very unpopular. It has been years, I think, since I have seen one.

$2 bills remain unpopular as noted due to a lack of familiarity among most Americans. There have been famous cases where people have been accused of (and even arrested for) attempted fraud when trying to spend them! Also, in some parts of the country there is a persistent myth that the bills are "tainted" or "cursed", which of course has no real basis in fact but may be due to the denomination's use many decades ago at race tracks and in brothels.

Two varieties of $100 bill were printed in 1914. As of 02/2015 approximate prices are:

With a blue seal

Circulated: $350 to $500 depending on the bill's condition

Uncirculated: $800

With a red seal

Circulated: $1000 to $2000

Uncirculated: $4000

Serial Numbers

A bill's serial number is a counter and a security feature. In most cases it doesn't affect a bill's value or help to identify it. Some collectors specialize in bills with low serial numbers (e.g. 00000005) or unusual patterns (12344321) so these can bring above-average prices.

DISCLAIMER: The values quoted are market averages as of the date shown, but may be different for an individual bill due to variations in quality and other factors. Also the wholesale (buying) price of a bill will be less than the selling (retail) price. A reputable currency dealer will be able to give a more accurate valuation based on an in-person inspection.

In general, their value is $2. I often get some from the bank, and have found that they will pay for exactly $2 worth of goods or services.

More

Many people think US $2 bills are valuable because they're not often seen in circulation. But contrary to popular belief, the denomination hasn't been discontinued and hasn't been withdrawn. They're still being printed, but in comparatively small numbers. Even so, tens or even hundreds of millions are printed for each run so modern ones are never going to be rare.

In some rural parts of the country people believe $2 bills are "evil", "cursed", or "bad luck". Those myths came about during the late 19th century when the denomination was heavily used at race tracks and in brothels where, in those pre-inflation days, $2 was the most common rate. In reality they're nothing but ordinary bills, no more "bad" than $1 or $5 bills and a LOT more interesting.