This is µCity (also spelled ‘uCity’, pronounced ‘micro-city’), the open-source city-building game for Game Boy Color. This game is completely free. Just download the ROM, install a Game Boy Color emulator, and play! The last release of the game should be here:
You can also play on real hardware. Even though the game has been developed using mainly emulators, it has been verified to work on hardware. The game detects how much available space there is and adjust the maximum number of saved cities accordingly.