Flarm is widely used by helicopters especially in the European Alps
(e.g. all Swiss SAR helicopter use it). It does't just work for
avoiding collisions with another Flarm equiped aircraft, it also has
an extensive obstacle database that will alert you of a potential
collision with e.g. a high voltage power line, radio tower or cable
car cable (all of which pose a very significant risk in the Alps).

Markus