Using an unsecured Wi-Fi network. There is disagreement whether Wi-Fi squatting constitutes illegal access to a third-party's network. However, the presumption is that when people walk down the street and find an unsecured network, they consider it intentionally open for the use of a store's customers and have little compunction against using it, which is generally for a brief period.
In a residential apartment building however, there are often open Wi-Fi networks within range of every other apartment that remain open for months on end. When novice users set up their networks, they often do not change the default, unsecured configuration in their wireless routers. See WEP