Starcraft 2 Arcade mode is now free for all users. What you need to do is to install Starcraft 2 starter edition, login with your battle.net account and click the arcade button.
There are tons of maps, rpg, tower defense, puzzles, co-op etc.
If you remember this article from exactly one year ago, you will understand that Blizzard won’t release a new game tool for developers.
Starcraft 2 Arcade is the tool and place where developers will create and place their maps. We will see if there will be any map that will create a new game genre as DotA did (MOBA).
Until then, download and install the Battle.net desktop client and grab Starcraft 2 if you haven’t done it already.