I use the Alaska Airlines debit card issued by Bank of America and linked to my Bank of America checking account fairly aggressively in order to manufacture Alaska Airlines Mileage Plan miles, earning 1 Mileage Plan mile for every $2 I spend on the debit card, including PIN-based and signature transactions.
I consider Mileage Plan miles to be one of the most valuable airline currencies (although I credit my Delta flights to my Delta Skymiles account in order to retain valuable Medallion elite status), since they can be used for one-way award tickets on Alaska Airlines and American Airlines flights (including "last seat" availability on Alaska-operated flights), and they allow you to combine one Delta-operated leg with another operated leg by Alaska or American, something you can't do with Delta's own Skymiles.
There is a lot of mistaken speculation (for example, in this flyertalk thread) that Bank of America no longer issues the Alaska Airlines debit card to new customers, so I want to make sure my readers are aware that you can still apply for the card. I first heard about the currently working link from Gary at View from the Wing, who heard about it from Free Frequent Flyer Miles.
So, if you have a Bank of America checking account, you can apply for an Alaska Airlines debit card here.
You can generate lots of easy, free, and valuable miles by using your Alaska Airlines debit card to fund Venmo transactions, load Bluebird and Gobank at Walmart, or pay other bills that only accept debit cards (although some transactions, like tax payments, may not earn miles).
One final note on the Bank of America Alaska Airlines debit card: unlike co-branded credit cards, your Alaska Airlines miles do not post after your monthly checking account statement closes. Rather, the miles are issued at the beginning of the month following the miles-earning debit card activity. I'll typically see my miles post to my Alaska Airlines Mileage Plan account between the 5th and 10th of the month, for the preceding month's debit card transactions.