Personal favorites:

- AirTran (RIP) "Wendy's cups for tickets" scheme in 2006. I was buying hundreds from dumpster-divers on Craigslist and giving everyone in my family 32 flight credits (worth 2 RT tickets). It also got me to "lifetime AIrTran elite status", a program they revoked the following year.

- Travelers Advantage (RIP) in 1997-2001 or so. A travel agent that would rebate 5-10% commissions back to you in the form of a monthly check. I was getting $1,000+ monthly rebates, as I was on the road 200+ days a year.

- ValueAmerica.com (RIP) sale of gift certificates at 70-80 cents on the dollar, depending on how many you would buy. I would book my travel with Travelers Advantage and then pay at checkout with heavily discounted Hyatt gift cards. Another $1,000+ per month in cash back.