Are you a commuter? Do you spend a lot of time in your car/truck/race van? If so, one of the best investments you can make is upgrading your car stereo with a KICKER Solo-Baric-Loaded sub box. They claim to be the best, loudest bass for plug-and-playing music in your truck or car. Generally found for $300 and up.
If you're not a commuter, then you probably spend a lot of time at home. KICKER got you covered there, too, with their iK500 “iKICK” Docking System. This thing has a genuine reflex subwoofer for superior bass and sound and runs in the neighborhoold of $249.
Both of these fine products can be found at kicker.com, target.com, circuitcity.com, homedepot.com, crutchfield.com, all Fry’s Electronics stores, and any Authorized KICKER Car Audio Dealer.