When WilliamHill.co.uk (a gambling site) launched in the UK they offered a promotion: a 50% rebate on your first stake (max 50 GBP)

The hack was simple, and spread like wildfire on campus:

  Place a GBP 100 bet on a basket ball game, at odds of 1:1.9. Get GBP 50 rebate.

  Get a buddy to take the opposite bet, at GBP 100 (they get GBP 50 rebate)

  One person wins 0, the other GBP 190. Split the 190, both parties are up GBP 45. 

  Repeat as many times as you have credit cards numbers (new registrations each time)

Lots of people pocketed hundreds of pounds in ~1 hours work, risk free, and we were students, so it was good money. I think it was 1999, maybe 2000.