I have a GW battle mat for rural/forest fights, but I wanted to do urban fights too. I wasn't sure how to make something that looked like good asphalt, but then I came across Krylon "Make it Stone!" spray paint:
http://www.krylon.com/products/make_it_stone_textured_paints/
I bought a couple cans of Black Granite Make it Stone paint, as well as some grey primer, and voila, it turned out quite well! It creates a nice, bumpy, textured look: