KaratGold and the KaratBank ecosystem has a referral program that seems to have attracted a large number of participants, with some even opening their own websites just to advertise the product.

However, the project has been accused of potentially being a multi-level marketing scam, so you'll need to do significant research to see whether it's worth your time or not.

Otherwise, check out crypto-games.net (see signature), they have a referral program where you earn a fraction of your referral's bet amount.