Warmill-Gaming- A really nice sci-fi fortress kit for about a 100 bucks
War torn worlds terrain is made from recycled tires, and comes pre-painted. Reasonably priced and virtually indestructable!
Hirst Arts the ultimate cheapskate terrain site, these guys sell terrain molds, so you can pretty much make whatever you fancy
Ironhands more of a custom terrain builder than a "product" but really nice stuff, and one of the few Ork terrain
sites
Armorcast Back in the day these guys had a license to make 40K vehicles and titans from GW, they produced the first grav tanks, bane blades, warhounds...now they stick to really good, and reasonably priced terrain. Their sci-fi stuff is ok, but what really stands out to me is the fantasy terrain (peasent dwellings for 13.00 bucks) and the egyptian terrain.
Terranscapes
Table top world- Fatnastic fantasy terrain. Clearly the Mercedes Benz of Fantasy terrain
FOW:
http://crescent-root.com/index.html
