Right now American Airlines has an interesting promotion called Miles for Milestones with either no downside or a pretty strange downside. For the 35th anniversary of their AAdvantage program they are offering 35 people a chance to win 350,000 AA miles to use on any of their routes or award partner routes. In addition, each entrant earns 350 miles just for signing up.
The upsides: you could earn enough AA miles to go round-trip first class for free to anywhere in the world. Or you will definitely earn a small amount of miles to help you get to your next domestic flights.
The downside: in the terms and conditions at the bottom of the site, it lists the “total ARV of each Prize [as] $6,965.” This may sound like a windfall but it’s possible they will send you a 1099-MISC form requiring you to pay taxes on the “income” you received as part of the prize.
I would still recommend signing up for the promotion anyway because the benefits far outweigh the costs. Imagine spending 25k miles on each roundtrip flight inside the US and you would earn 13 free flights with this prize. I signed up for it this morning.