Hostgator allows you to create your own discount coupon codes and if someone uses your coupon, you get a commission. They even don't have to click on your affiliate link, just use your coupon. Or even if they click on someone else's affiliate link and then use your coupon, you get the commission. I wonder if there are any other web hosting companies that allow you to create your own coupons? I know only about ASmallOrange.com web hosting, but they don't pay as much as Hostgator. Do you know about any other?
There are lots of hosts that would probably be willing to create coupon codes for you, whether or not they'd attribute it to the affiliate commissions is another question. Likely you'd be best to contact hosts you're interested in about this since not a lot of them will have this with any form of automation or have it announced that they offer what you're looking for, and only upon asking them will you get any kind of actual answers.
Thank you for your suggestion. However, when using a Dreamhost coupon, the referral also has to click on your affiliate link to qualify as a referral, or not? (check here: http://dreamhost.com/affiliates/)