Some airlines offer Y-up fares (typical for domestic USA) for elites. Buy a full-fare coach ticket, get an immediate confirmed upgrade to first if a first class seat is available at the time of purchase. These fares still show up as a "coach class" purchase.

This is great for people who are not allowed to buy first class tickets (company policy), yet it still allows the airline to sell a high price ticket. For example, on CO, many Y class coach tickets are only $50 less (one way) than full fare A class first tickets.

It's also an incentive for elites. Again, with CO, on some routes their Y fare is the same as WN's (Southwest) refundable ticket price. An elite on CO would have a very strong incentive to buy a CO ticket and get first class, instead of coach on WN.

This is a little different from international business class upgrades, which are never free (that I know of). Internationally the best "free" (no extra miles, certificates, or money) "upgrade" that one can get is premium economy (VS, NZ, BR, UA sort of, etc.)