StarCraft 2 unlocks free-to-play options, but it’s not completely free
This is free* with an asterisk, however; only the Wings of Liberty single-player campaign, all of its multiplayer content, and co-op commanders are included in this statement. The other three single-player campaigns have to be purchased separately**, although this comes with a double-asterisk to indicate that those who previously purchased Wings of Liberty can get Heart of the Swarm for free through December 8th.
Some players have noted that this isn’t technically the first time that StarCraft 2 offered a free-to-play option. The StarCraft 2 Arcade, which includes a fan-made MMO mod of the game, has cost nothing to use for quite some time now.