SpotPass (CrossPass in Japan) is a feature available on every Nintendo 3DS system. With it, players can connect to several designated Wi-Fi hotspots and automatically download game content and system updates, even when the 3DS is in sleep mode. When it's being activated, the 3DS system will indicate this by flashing a blue light on the corner of the hinge.
Nintendo of America in the United States managed to make a deal with AT&T who agreed to provide free Wi-Fi access to 3DS users at 10,000 different hotspots throughout the country. The service was initiated in May of 2011. SpotPass is used by some games to download DLC.