This might seem one for Londoners, but i don't think this applies strictly just to them.
Barclaycard are giving £25 credit when applying for their new Barclaycard OnePulse Oyster Card and making your first purchase.
So buy a pack of Gum, or something, and £25 is yours. Also comes with the handy one-touch payment method and doubles up as an Oystercard, although i'm not sure i want to flash my credit card around to everyone when i'm going through the gates.
Also has 0% interest on payments for first 6 months.