If you mean great inductive loads, you need read some about reactive power , reactive power compensation. May be it's no the better article :**broken link removed**
If the power factor is negative then the average power is flowing the wrong way. As Sceadwian says, the load is a generator (or at least something capable of supplying power).
e.g. your electricity company supplying power to your house would see a negative power factor if you have a solar supply and it is supplying power to the grid.
Alphacat, any electric motor under the right conditions can produce energy rather than use it, so any electric motor as a load in theory can produce power rather than use it. Under any normal circumstances however motors draw power not generate it.