I have a sheet metal nibbler that attaches to a drill motor. I recall it cost me $25 on Amazon. I also have a sheet metal saw from HF like Charlie suggested above.
The nibbler is great for cutting irregular curves. Here's a picture of it.
] I also used it to do the cutout for this patch
] in my fender.
But when using it you have to watch out for the sharp little crescents of metal it leaves behind. The Missus won't want you tearing up her floors when they stick in your shoes.
I bought a plasma cutter also (one of the cheapo ones supposedly rated for 1/2 inch steel thickness), but rarely use it because you need to setup the plasma cutter, connect the compressor up, etc. etc.