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.