I use air shears on thin (up to 1.2mm) sheet metal- brillant for 8x4 sheets. only cost £30 so can't complain. Burr free aswell so alot safer. I have pretty much every cutting tool mentioned in this thread including a plama cutter. IMO If you don't do a lot of cutting and are short of space the cheapest option is a 9" grinder. Even with a large bandsaw and plasma I still use grinder a hell of a lot.VERY versitile!
If you don't have a compressor you can get a nibbler which attaches to a drill. The Falcon Nibbler is made in the Uk and is good quality. Go to www.toolsbypost.com and put nibbler in the search and you will get plenty of choose from. In the air tools section you will also find several air tools for cutting sheet steel.