If you fancy a rather funky looking bit of plastic that could make a nice generator or power station, buy a pack of these:
http://www.sainsburys.co.uk/shop/gb/groceries/simply-turkey-burgers-454g
The plastic tray they come in looks like this:
Which when flipped looks like this:
Fairly cheap, and you get the bonus of some actual edible food with your new terrain!
I know we all make terrain from junk, but this is the only piece I've seen turn up with my shopping that looks like it could make something rather cool looking with very minimal effort. the plastic itself it actually quite sturdy too, so could easily support a good few models and won't need any sort of reinforcing before painting.
I'll update this thread if i find any more goodies or when (more likely 'if') I actually get round to painting this up.
**Other brands of supermarket are available**