I've got a Blaupunkt THB200A (no longer produced, but can be picked up second-hand) which is mounted behind the passenger seat.
It's never going to push the windscreen out with bass but it's surprisingly effective.
The below JL slimline sub gets great reviews, and has been mentioned on here by an audio guru once before (it's how I came across it), I am considering it to try and fit between my drivers seat on the floor and the beginning of my run of furniture..
Ive had a clarion SRV313 underseat sub for about a year in my smart fortwo, for its size it packs an impressive punch, got it from wilco for about £90. it has a wired remote controller for the gain and frequency, i hid it under the seat so as not to draw attention to it