Since these little sub systems came out a couple of years ago, we have sold dozens and dozens of them at my shop. At a small size of 19" X 11" X 5" and a light weight of 17.5 lbs, it is a great problem solver if you don't have a ton of space. PLUS, because they are ported, they pound pretty damn good too without needing lots of power.
Here is some more info on them:
CP108LG-W3v3 - Car Audio - Subwoofer Systems - FlatWedge? - JL Audio
If you have a small car like an S2000 or just don't need a box taking up your whole trunk, this little beast can get the job done at a pretty decent price too. At $489 its a great value!