Having too much time on my hands, I've had a closer look at Citibank's website. They have a 'Special Offers' page, where you can enter codes and it then redirects you to a page with some info about the offer and an application form. How does the page decide where to redirect someone when they enter a code? Why it runs a piece of Javascript in the user's browser that contains one huge 'switch' instruction with a 'case' for every code. A little bit of dissection and you have all the codes. So I've made a list of the more interesting ones, though I certainly haven't had a chance to examine them all, there's hundreds, but a lot seem to be repeats of the same offer, probably with different codes to track where the user got the offer from. According to the comments in the code, some are quite old as well.
The page to enter the codes:
Citibank Australia
PDPA7WYA : Platinum Card, 3.9% on life of balance transfers
DD4872DR : Gold or Silver, 3.9% on life of balance transfers
PDP772DB : Platinum Card, 2.9% for 18 months on balance transfers
DDN872DE : Gold or Silver, 2.9% for 18 months on balance transfers
DUP96ZZB : Gold or Silver, no fee for life
43FA4HYA : Clear Card, 4.9% on life of balance transfers and no fee in first year
P3P152YB : Platinum Card, 2.9% on life of balance transfers and no fee in first year
D3P152YG : Gold or Silver, 2.9% on life of balance transfers and no fee in first year
D3NA52KA : Gold or Silver, 2.9% interest rate for first 6 months
Wether any of the forms attached to these offers still work is left as an exercise for the reader, they all at least bring up a page with the details and the "Apply Now" button.
In my travels I've also come across two expired offers, one that gave away 5000 QFF and another that gave away a $50 Holiday Inn voucher, so maybe such offers will pop up again in the future.