In Huckle Buckle Beanstalk, the player must guess the random number between 1 and 100. The player gets feedback after each guess to help guide them in their next guess.
Although the game looks simple (and it is fairly simple, from an interface perspective), much thought went into the user experience. It neatly encapsulates the way I think about design in terms of how it affects the user.
The user may input a guess either by clicking the on-screen guess button or by pressing the enter key. Once a guess is entered, the feedback is displayed, the guess field is cleared, and input is focused back on the field making it quick and easy to enter the next guess.
Another small touch is that the game tells how many guesses it took for the player to guess correctly. This facilitates a competitive drive to win in the fewest guesses.
Take a look at the code to see how the game was built.