there are many games playable by 2 players... it just depends on what you are looking for...
Your question is very hard to answer without more details about the length of game and depth of strategy you want to get into.
many of the games listed on "most popular game lists" on boardgamer sites are playable by 2-4 players... (though these sites seem dominated by the eurogame titles by rio grande/Zman/Avalon Hill/Mayfair Games/Fantacy Flight - and few to none of these are available at Walmart/Toys R us like your standard Parker Brothers Clue or Monopoly...although I have seen a limited selection at Barnes and Noble stores.)
Settlers of Catan Card Game
Agricola
Dominion
Pandemic
power grid
Race for the galaxy
Jambo
Memoire 44
of course there are the americanized games which are available at your local walmart
sorry
Uno
Sequence
Mille bornes
Scrabble
Boggle
Yahtzee
Definately need more information on your games preferences, to actually assist you...but I highly suggest you checkout
thoughthammer.com or
CoolstuffInc.com or start your search at
BoardGameGeek.com