Kicker 6.5" CompVT Car Subwoofer 2 OHM 10CVT652
This subwoofer was a definite upgrade for my car.  The bass is much tighter than before (granted, the original sub was damaged), and it seems you can drive it pretty hard with no ill effects.  I had to trim the plastic coating off to fit it in my Saab 9-5 wagon, but installation was otherwise straightforward.  I even called the 800 number to clarify the wiring scheme, and the service rep was very helpful.