For my subwoofer I selected an NHT1259 woofer. I've used it in another project and I was very happy with it. 

Select a subwoofer based on a few parameters. The most important are F_S (resonant frequency) X_MAX (maximum throw) and Q_TS (total Q). I want the resonance as low as possible, the maximum throw as high as possible and total Q appropriate for the box style I have in mind.

In the case of the NHT, F_S is 16.5 Hz which will produce an awesome low end. X_MAX is 13mm which is a lot, and implies plenty of sound output, and Q_TS is .445 which is appropriate for a sealed box (my preference for a subwoofer). 

You can use a vented box for a subwoofer. Venting produces higher efficiency and a deeper low end but it uses a much larger box and a vented woofer can have serious problems with excursion if fed frequencies below the resonance of the system (excursion grows dramatically below system resonance). A sealed box uses a smaller box and has no excursion issues but it will not go quite as low and it does have lower efficiency. The NHT has plenty of sound capability thanks to the really long throw so I'm not worried about efficiency that much.

Here's a picture of the woofer. Click it for a larger view. It's a 12" diameter woofer with a very big magnet.