Depends on the kind of cut you're doing.
All-purpose: Dremel with fiber-reinforced metal cutting attachment. Dremels will technically work for anything, but they tend to create a more ragged edge and mess up the paint in the area more than other stuff.
Straight cuts, some curves: Jigsaw with fine-toothed metal-cutting blade. I haven't had much luck with this personally but I probably just have a bad jigsaw, or I'm not doing it right. Or both. Other people recommend this highly though.
Circular holes (for fans usually): Hole saw, haven't used it personally but I'm sure it's the best solution for this.
Straight cuts: Skilsaw/circular saw with metal-cutting blade. These are supposed to be the very best for a straight cut, and should provide a very clean, straight edge.