Crazy Eights, Spit and Gin all appear on Mayday Games' list of two-person card games. Pagat has an extensive list of card games that can be played with two people. On this list are Blackjack, Cribbage, Hockey and Last One.
Gin, also known as Rummy, refers to a group of card games in which the basic goal of gameplay is to match cards of the same suit, sequence or rank. Some common variations of Rummy are Block Rummy and Children's Rummy. Blackjack is one of the most widely played casino card games in the world, and can be played with two or more players. Wikipedia has a list of card games that require only two people to play.