Not "free" per se, but a couple of credit cards include Global Entry fee reimbursement as one of their benefits. IIRC Amex Platinum and Citi Prestige are two, and there may be more that I'm not recalling offhand. I wouldn't get one of those cards just for the reimbursement but if one of those cards is a useful product for you it might be part of your value calculation.

If you happen to be a US military member or DoD civil service employee, you're already eligible for Pre and just have to use the number from the back of your CAC card as your known traveler number.

Delta Platinum and Diamond elite statuses include Global Entry application vouchers a benefit.