What are the essential Starcraft 2 maps for training?

Multitasking Trainer

On the bottom left side of the map is your starting position and right above that is a terran base. Your purpose is to create an army and destroy the enemy base. The terran will attack and/or drop you at several points in time depending on the difficulty setting. There is also a neutral unit at the island on the right side that you have to rescue and bring back to your base.

The tricky part is that in the top right side of the map you control a probe that is constantly being chased by a zergling. So you need to manage your base, defend the attacks and save the neutral unit, all while microing the probe to keep it from dying. This is a great way to practice not only your early game scouting but also your multitasking and hotkey usage.

Yet Another Build Order Tester (YABOT)

This project is a series of standard 1v1 Blizzard maps adapted to include a sophisticated build order testing system. You are given 7 minutes to execute the chosen build while keeping up worker production and creating the largest army possible. You will need to pay some attention to Micro because the AI will send attack waves depending on what build order you choose for the computer. At the end of the 7 minutes, you are given a score which tells how well you executed the build. Also included is a "Freestyle" build, which monitors your play and displays the build order you performed.