If you're looking for a quick "pick up and play game," give iHangman a try? iHangman [iTunes link] allows you to play the timeless game of hangman right on your iPhone.
In iHangman, you pick letters from the side to make a word. If you don't get the word after a couple tries, then you complete the hangman and lose the game. However, if you guess the word, then you win. iHangman gives you plenty of options in the Settings tab -- you can change the timer, have single or multiple players, see the leader board for your iPhone, turn sounds on/off, and choose the word list categories. You have the option to choose from a myriad of word lists including: Animals, countries, sports, SAT, GRE, Science, and more.
This game includes over 36,000 words and their definitions. That's right ... definitions! When you lose or win a game, you are presented with the full word; when you shake your iPhone side-to-side, you will see the definition of the word. This is a very nice feature. Overall, iHangman is a fun game to play, and at $0.99US, it's not very expensive. You can download this game from the iTunes App Store.