Blizzard has finally released the Starcraft 2 Demo.
Updated for new version of demo:
The StarCraft 2: Wings of Liberty demo has been replaced by StarCraft 2: Starter Edition. The new version allows players to play four levels of the campaign, the first two challenges, access the terran race in custom games and single-player vs. AI, and play four of the custom maps.
All campaign progress and achievements are saved to the player's Battle.net account, so they can pick up from right where they left off, if they decide to buy the full game. Already have a Battle.net account? You'll find the StarCraft 2: Starter Edition in Account Management under "Your Game Accounts."
Here is the link to the free demo.