As you might guess, it's "W". The W mint mark is only used on commemorative and bullion coins.
Interestingly, West Point also struck some circulation cents from 1973 to 1986 but only on an as-needed basis when the Philadelphia and Denver Mints were overloaded. Those cents do not carry a mint mark so that they are indistinguishable from Philly coins. The reason that's done is to avoid creating instant rarities because West Point's capacity is much smaller than the other 2 mints.
The U.S. Mint, of course. Currently cents are minted at Denver (D below the date) and Philadelphia (no letter) In the past, cents were also made at San Francisco, with an S. When the Phila. and Denver Mints are overloaded, the West Point Mint sometimes helps out. It doesn't use a mint mark letter when it makes pennies, so they look just like Philadelphia ones.
If you mean mint marks, cents have only been issued at Philadelphia and West Point (no mint mark from either mint), San Francisco ("S") and Denver ("D").
Pennies are minted at three locations in the US; The San Francisco Mint, The Denver Mint and the Philadelphia Mint. Pennies from San Francisco have an "S" on them. Pennies from Denver have a "D" on them and pennies from Philadelphia have no letter.
There are no tours at the West Point Mint.
West Point Mint was created in 1937.
The letter indicates the Mint that struck the coin. Mint marks appear on all current U.S. coins except for cents that were struck at Philadelphia, PA and West Point, NY. See for example http://www.coinworld.com/news/012306/bw_0123.asp
Philadelphia, Pennsylvania. Philadelphia is the only US mint to not put a letter mintmark on the coins made there. [ The mint at West Point has , at times, struck coins for the Philadelphia mint with no mintmark]
The letters under the date on some pennies indicates which US Mint branch made the coin. The letter "D" indicates the Denver, Colorado Mint, the letter "S" indicates the San Francisco, California mint and no letter at all indicates the coin was struck at the US Mint in Philadelphia, Pennsylvania.
The mint at West Point began by striking cents from 1973 to 1986. These cents had no mint mark and can not be distinguished from those struck at Philidelphia. In 1988 West Point was officially made a mint and today the mint mark "W" appears on coins struck there.
No Mercury head dimes were struck at the West Point Mint. The "W" on the front of the coin is the designer's initial.
Platinum Bullion coins are produced at the Philadelphia Mint and West Point Mint.
On a US coin it's the mint mark of the West Point Mint. Normally West Point only manufactures special collectors' coins such as commemoratives and bullion pieces, but in 1996 it struck Roosevelt dimes for the coin's 50th anniversary. Be careful to distinguish the W mint mark from the same letter on Mercury dimes and Walking Liberty halves. On these coins it's the monogram of their designer, A. A. Weinman. In any case they were struck decades before the West Point Mint was opened.