Adire Tempest
http://www.adireaudio.com/car_audio/subwoofers/tempest.htm
Shiva
http://www.adireaudio.com/car_audio/subwoofers/shiva.htm
brahmas
http://www.adireaudio.com/car_audio/subwoofers/brahma_series.htm
Some Blueprint subs
http://www.blueprintdrivers.com/ (their site is currently down)
Stryke HE 15, as well as several of the Lambda offerings
http://www.stryke.com/ (the slight shelving is caused by the low Q motor)
Ground Zero plutonium
http://www.magma-audio.de/english/gz_plutonium.htm (shelving is caused by the low Q motor)
I believe the Peerless XLS drivers will do it
http://www.d-s-t.com/main/index.htmThese are some... Some of these I have tried ported, some of these have not.
The best woofers for a flat to 20 Hz box will have a super low Fs and usually a fairly high Qts. Don't let high Qts discourage you from porting, they can still sound excellent.
Adam