Two be sure, three of these eighteen bridges don't exist yet and one is still under construction, but I agree that the ones this site has chosen are pretty cool.
I can only say that I've actually been on two of these bridges: Tower Bridge in London and Erasmusburg in Rotterdam. I've also seen the Akashi-Kaikyo, albeit from a bit of a distance:
Believe it or not, I have yet to make it out to San Francisco to see the Golden Gate. Yes, I'm that lame.
(hat tip: Marginal Revolution)