+1 on the hand-held shears: I went through a variety of sheet-metal cutting solutions.
I started with a wood working jig saw. In retrospect, this is not a terrible solution with the right blade, which is very, very fine tooth, 24tpi or better. The nice thing about this is that they're made to use with a clamp-on fence, so you can make decent straight cuts.
Scissor type shears & aviation snips -- works and well if you get the hang of using the aviation snips, but you need a grip like an ape. Cutting a straight line isn't their forte, but curves are good. I'd say they top out at about 18 ga.
Sawzall -- ugly cuts, hard to hold the work, loud, beats the h*ll out of the material, etc. but still earns it's keep on demolition work and structural steel.
Chinese 3-in-1 machines. I like the slip rolls and press-brake, but I could never get the shear adjusted to work well. It worked OK on 18 ga. aluminum, but thinner or tougher stuff just slipped between the blades which would flex.
Beverly style shears. These are *sweet* for certain jobs - like the aviation snips, they do well on curves, but don't do so well on straight cuts. My example has a 1/2" bar cutter, and you'd really have to eat yer wheaties to cut that, but it'll happily cut 3/8" all day long, even stainless. That's a nice feature if you do lathe work on small parts.
The shears that dsergison show are electric. They come in pneumatic as well, but that, of course, requires a compressor. They do OK with a fence for making straight cuts, but aren't exactly designed for it. They're a really good, relatively low cost solution to cutting sheet. Don't skimp on these tools - a cheaper one will have lower capacity and the shearing blades dull quickly. Kettler (sp?) is a good brand.
Regarding a "stomp shear", if your work requires really nice, straight edges, for example, to butt-weld with no additional dressing, these will do the job, but take up a lot of floor space, and they're pretty heavy. They won't do curves, either.
Forgot to mention: plasma... OMG, it's cool, but the cuts are ragged, and require dressing before welding since there is a bit of slag on the back. The best use for plasma, IMHO is stainless, which is hard on cutting tools.