Xplod is good. CDT Audio and JL Audio also make great subs. I've got a CDT Audio 10" sub mounted in the rear-door spare tire space, in an enclosure custom-built for me by Shawn from this forum (username = psychos2). I love it!

I bought a 400 watt Blaupunkt subwoofer amp from Fry's, on sale, and I'm completely happy with the sound. I didn't change my stock speakers; just added the amp and the subwoofer. The system sounds a million times better now.