Exploding Creatures
Left click any creature several times to make it explode.
Unlock Hidden Dialogue
Keep left clicking units to hear secret dialogues.
Space Shooter Mini-game
Click the arcade machine on the Hyperian to play a space shooter mini game.
Ending Sequence
Unlock Alternate Background
Complete the Campaign.
Secret Mission: Piercing the Shroud
Head down to the far bottom right corner of the map. There's a civilian building on a little floating platform. Destroy it, and "secret documents" will be revealed. Pick these up with any ground unit, and then complete the mission as normal. After, you will now have the secret mission available for selection from the Star Map.
"Havens Fall": Marine
In single player campaign, search the lower left corner of the map near the refugees. You can find a frozen Marine just before the bottom left Zerg base in the water.
"Piercing the Shroud": Marine
In the loading bay before the room with the Brutalisk, go outside through the door. Follow the rock ledge as it leads up and more of the map will be visible. Continue until you find a Murloc Marine.
"Welcome to the Jungle": Marine
In single player campaign, you can find a Murloc Marine near the left edge of the map, slightly north of your base. He is standing on a cliff edge that is surrounded by trees.
"Zero Hour": Marine
In single player campaign, search the top of the map just above the left Vespene geyser. You can find a Tauren Space Marine that enters an outhouse. Repeatedly click the outhouse and it will shoot into space.
Lost Viking Mini-game
Click the arcade machine at the Cantina on the Hyperion to play a space shooter mini game.
Zerg Queen Quick Larval Injection
Select all queens with "control group". Hold [Inject Larvae] then press [Backspace] to jump between each hatch and click that hatch. This will use the closest queen to cast "inject larvae", without having to press [Inject Larvae] each time.
Blow Up Creatures
Repeatedly click a creature and it will explode.
Hidden Dialogue
Repeatedly click a unit to hear additional dialogue.
Blizzard References
During the Protoss missions, watch the monitors to see a music advertisement for LVL800ETC, featuring a lot of songs. This is a reference to the metal band, LVL80ETC, which members are Blizzard employees.
Successfully complete the "Ghost of a Chance" mission and unlock the Ghost rifle. Go to the armory on the Hyperion. Click the Ghost rifle to examine it. Keep looking at the screen until Nova (from Blizzard's unreleased game Starcraft: Ghost) briefly appears and disappears.
Diablo Reference
During "The Devil's Playground" mission, on Redstone III go to the bottom right corner of the map. You will find Diablo, from the game of the same name, surrounded by lava. He will attack your units when discovered.
World of Warcraft Reference
Enter the ship's cantina. Look at the upper left to find a red female holograph that is doing the identical dance as the female night elf from World Of Warcraft.
Unlock Juke Box Songs
Terran Up the Night - Beat the campaign on any diffuculty.
Cow Chat Channel
When in the Starcraft 2 menus press the blue symbol under the title. This will place you in a public chat channel called cow. Every time you speak your message will be followed by ...moo!