The sub I had in my 04 Neon was the Dayton Reference HF 10" from PartsExpress. I highly recommend this sub for soundquality at a great price. It was hard to distort and offered down to 20hz and beyond in a mid-sized sealed box (I want to say .8 cubes, but I can't recall for sure). Here's a link to it...
http://www.parts-express.com/pe/show...number=295-460
Just a tip for those who want an efficient subwoofer... one thing to note is that a lot high-end car audio subwoofers aim for huge amounts of power handling anymore. To do this, they tighten up the sub, causing it's efficiency to plummet. Not all high end subs do this, but if you see "2000 watts RMS power handling", chances are, it isn't going to do very well with little power and isn't going to be very efficient.
I'm still trying to research a sub to use in this car... though, I put in Parkway Drive the other day and the low end punch on the kick was outstanding! Must be centered around my ~60hz peak. Anywho, finding the right size subwoofer that is light-weight (neodymium magnet structure is most likely required) with a very efficient 1watt/1meter rating is going to be tough, but I'm in no hurry. I'm just too picky about that "perfectly flat response"... even though there isn't such a thing in a car!
Car stereo is just a beast to get sounding good compared to home audio.