Black poster board, $.59 for mat, $1.29 or so for shiny. Pretty big, good for at least a 29g tank. Really cheap. I've painted, bought backgrounds, used trash bags, and this, for me was the most economical without sacrificing too much of the look. Trash bags took me forever to straighten, and what if you want to change the paint? Emulsion, I don't like getting my hands all painty, but to each his/her own.
For a larger tank, probably poster paper. A little thinnish, but still a very cheap option, if you like changing things a lot, like I do, and don't like scraping paint, which I despise.
have heard of people using crushed velvet. Expensive, but could possibly look cool, never tried that. Other fabrics may be an option, since you can wash and dry those.