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. It is also possible to get 1,000 bonus points; this is achieved by catching a normal and bonus nasu at the same time.
Konami Code
By performing a button combination nearly identical to the Konami Code (left, left, right, right, up, down, up, down) in Nasu's title screen, the player character's head is replaced with an eggplant (aubergine). Doing this seems to greatly improve the chances of a bonus nasu. While this is likely a reference to the Konami Code, it may also refer to Kid Icarus for the Famicom, where some enemies transformed the player into an eggplant with legs.
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A Famicom is a rather outdated console by today's standards, but it isn't unusual for someone to own and enjoy using one. However, other things in Madotsuki's room seem a bit outdated, such as the TV itself.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. Alternately; it could be that because she never leaves her home these items represent the time period when she changed and stayed indoors, hence nothing is updated or new.