NASU is the horribly depressing famicom game found in Madotsuki's Room. The graphics and sound highly resemble authentic Famicom/NES hardware capabilities, although the game itself is fictional. It is possible that Nasu wasn't the only game intended to be inlcuded in Yume Nikki, since it's a member (yet the only one) of the Famicom menu.
Gameplay
Nasu is a very simple arcade game in which the object is to catch a falling nasu (eggplant) in the player character's mouth. Failing to catch a nasu will result in losing the game. Each nasu is worth 10 points, and occasionally a bonus nasu will bounce from the left side of the screen to the right and is worth 300, and does not result in a game over if not caught.
Fan Speculation
Toriningen
The player character of Nasu highly resembles the Toriningen of Madotsuki's dreams. With its depressing nature, the game could be influencing her dreams negatively in this manner and many others.
Time Period
A Famicom is a rather outdated console by today's standards, but it isn't unusual for someone to own and enjoy using one. However, other things in Madotsuki's room seem a bit outdated, such as the TV itself. This may indicate that the events of Yume Nikki take place sometime in the recent past.