Free £5 Tube (London Underground) credit
American Express cardholders can currently get £5 free credit on the London Underground by registering here.
The £5 credit will be credited back to your American Express card when you spend £5 or more using your card as the “contactless” payment method at tube (or contactless-enabled National Rail) gates.
I managed to register my UK GBP American Express and my Euro International Credit Card American Express for this offer, so it does not appear to be limited to GBP-only cards.
(HT: Headforpoints)