During Blizz Con, Blizzard announced that Starcraft 2 was becoming free-to-play, and today is the day that happens. The game is no ready to be downloaded from Blizzards own service, free of charge. There are some circumstances to this though.
The free part of the game includes the entire single player of the first campaign, Wings of Liberty. It also includes limited access to multiplayer which includes full access to unranked matches and custom games. However anyone that wants to play in ranked games will need to meet some requirements. You first need to win 10 matches of unranked games, and will have some level caps. All commanders are free to use up to level 5.
This included a newly released Terran commander combo Mira Han and Matt Horner.
Anyone that had already purchased Wings of Libery by October 30th can get a free copy of Heart of the Swarm until December 8th. There are two more campaigns to purchase as well including Legacy of the Void®, and Nova Covert Ops. Both can be purchased for $15 each, or in the collection of all expansions for $40. Anyone that has already purchased any part of the game before October 30th will get a special Eidolon Ghost skin and three portraits commemorating their founder’s status.
You can read more in the official release.