This is bold. The part I already knew: you can now play any of StarCraft 2's three races in multiplayer for free. The part I didn't expect: you can also play any of the user-created custom maps and modes available in StarCraft Arcade for free, too. That's a huge amount of free game, and a huge boon for the mod community.
Mod tools might be my favourite thing about PC games. They benefit everyone: players get new experiences to play, extending the life of their games for free; developers future-proof their games by surrounding them with a lusty pool of new content; and budding young developers get the resources they need to enter the industry, and to have an impact upon it even before they have.
Being able to make something for StarCraft 2, and then have it be playable by people who don't even own the game is marvellous.
Blizzard know the power of modding more than most, with Dota/DotA/MOBAs beginning life within their creative ecosystem. When I spoke to Blizzard president Mike Morhaime last year, he said his biggest regret was that they hadn't done more to recognise that mod's success. I imagine Blizzard are keen to stop that from happening again.
This also means that the only parts of StarCraft 2 you pay for are the campaign and ranked multiplayer. The free parts will act as a good advert for buying the rest, especially if you're enjoying the multiplayer, but really it means there's enough to it that you can pretend StarCraft is a wholly-free standalone game.
You can download the StarCraft 2 Starter Edition from the official site.