You could use darn near anything. Whatever fits in any particular area and cuts metal. It also depends on the nature of the cut, ie. curves or straight lines and how perfect you want things. Low end is tin snips and high end is plasma cutter. I've used jigsaw (w/ hacksaw blade attachment) grinders, cut-off wheels, sawsall, hacksaw, oxy-acetylene, nibblers or just pain old bending until it breaks off,
I prefer using air tools, but if not available I usually install a cut-off wheel on my cordless drill.