@Linkat,
In "common law" jurisdictions such as the United States, Great Britain and the British Commonwealth, there is no general duty to come to the rescue of another. However, contrary to common law, eight US states have laws requiring people to help strangers in peril: Florida, Massachusetts, Minnesota, Ohio, Rhode Island, Vermont, Washington, and Wisconsin. These laws are also referred to as Good Samaritan laws, despite their difference from laws of the same name that protect individuals that try to help another person. These laws are rarely applied, and are generally ignored by citizens and lawmakers.
Unlike the United States, many European civil law systems provide a far more extensive duty to rescue. Any person above legal age, who sees another human being in peril, must take all reasonable steps to provide help. The only exclusion is that the person must not endanger her/his own life or that of others, while providing rescue.
In theory, this can mean that if a person finds someone in need of medical help, she/he must take all reasonable steps to seek medical care. Commonly the situation arises on an event of a traffic accident: other drivers and passers-by must take an action to help the injured without regard to possible personal reasons not to help (e.g. having no time, being in hurry) or ascertain that help has been requested from officials. In practice however, almost all cases of compulsory rescue simply require the rescuer to alert the relevant entity (police, fire brigade, ambulance) with a phone call.
France has a law that people can be prosecuted for "non-assistance à personne en danger" (deliberately failing to provide assistance to a person in danger), which can be punished by up to 5 years of jail time and a fine of up to €100,000. It is not used very often, mainly for the reasons outlined previously by other posters - it would be unfair and unrealistic to
require people by law to put themselves in danger, but people are occasionally prosecuted.
The photographers at the scene of Princess Diana's fatal car accident were investigated for violation of the non-assistance law. In another case, which shocked the country, a woman fell in a river and drowned near a camp site and a number of the campers stood and taped her struggles with their camcorders. It was some while before anybody thought to use a mobile phone to call for help. When the police arrived they confiscated all the camcorders and played back the footage, and everybody who had filmed the scene was prosecuted. This met with widepsread public approval. She probably would have drowned anyway, and nobody was prosecuted for not jumping in the river.