Good news, everyone! Patch 2.1 has dropped for Starcraft and a grand hunk of content is now completely free to play! This includes the entire Arcade and Custom maps (as well as competitive team matchmaking as long as one person owns the game). The single-player campaign and 1v1 competitive matchmaking still require you to buy the game.
Check out their overview of Patch 2.1's features!
Also, they put together a fun video for map/modmakers :)