A huge chunk of StarCraft II is going free later this month as a sort of big 'ol demo for the wider trilogy. From November 14th, everyone will be able to play the full Terran story campaign from the original Wings of Liberty, the competitive multiplayer including units from all three chapters, and -- with some limitations -- the cooperative mode too. If you dig it, you'll be able to buy the other bits of SC2 separately or in one big lump.
Oh, and people who own Wings of Liberty but skipped Heart of the Swarm will be able to get that Zergy instalment free - if they claim it in time.
So! Free SC2, eh? Yup, Blizzard are turning the heart of it free, with one of the three big story campaigns, all the competitive multiplayer, and most of the co-op. There are, of course, restrictions.
Presumably to block cheaters, smurfs and other undesirables, ranked matchmaking will be locked until an account has won ten games on separate days in unranked play or against the AI. There's a little time-consuming faff to stifle people who'd abuse open free accounts, basically. As for the co-op limitation, commanders other than Raynor, Kerrigan, and Artanis will be capped at level five unless players pay. That's not so bad, seeing as half the game's co-op commanders cost money currently anyway.
If players dig the demo, they'll be able to buy the Zerg and Terran campaigns for $15 each, or they all come collected in the Battle Chest, same as they do now. Oh, and that Nova campaign is sold separately too, as it is now.
And that free Heart of the Swarm I mentioned? If you owned Wings of Liberty as of October 31st, you'll be able to grab Heart of the Swarm free between November 8th and December 8th. It'll be yours for keepsies but you will need to fire up the Battle.net client (yes, it is still Battle.net) during that window to claim it.
The big free lump is a pretty neat move. I already liked how StarCraft 2's Arcade was free, letting everyone play the many weird and wonderful custom maps and modes, and giving everyone full multiplayer is a great move.
Hit StarCraft 2's site for more information on how all this would work. Or watch this video: