Everybody knows UNO, the classic card game where you have to match cards going around a table and try to clear out your hand before someone else clears out theirs. Gameloft produced the iPhone version, and it's a respectable recreation of the card game that's available in single or multiplayer. The game even has some extra rule sets, so if you're tired of the core game, you can try some others. Unfortunately, there's no Game Center integration, but the app was updated for Retina Display recently.
In celebration of the game's 40th anniversary, it's on sale at 99 cents for a limited time, so head to the App Store and grab it while it's cheap. There's also a free version to try if a dollar is somehow too much, and there's an iPad HD version as well, though it's still at its normal US$4.99 price.