2,500 Delta SkyMiles for $8 Courtesy of Netflix
While this offer is technically only for "new" members only, you can generally get these bonuses as long as you use a different credit card, address, and email address.
If you've gotten a Netflix bonus with your Skymiles account in the past, you also may not be eligible, but it's worth a try in my opinion. This is a super cheap way to get miles - at this rate, a roundtrip business class award to Europe would only cost $320!
After your $8 for the digital membership is charged, I'd recommend waiting a couple weeks and then cancelling - hopefully your miles will have shown up by then. If not, don't worry - this bonus does not require a multi-month contract.
To be honest, I signed up for Netflix a while back for the US Airways Grand Slam promotion and I find the service to be excellent - you get the movies in a couple days and their selection is pretty impressive.
Tip of the hat to TPG reader Ted for this one!