It appears that in preparation of StarCraft II: Heart of the Swarm, Blizzard is offering the original game in this series, StarCraft II: Wings of Liberty, for half the standard price in both North America and Europe.
Wings of Liberty marked the first StarCraft game since 1999's expansion of the original, Brood War. Wings of Liberty reintroduced Jim Raynor, the love interest of Kerrigan, before she was assimilated into the Queen of Blades, the leader of the deadly Zerg armies. As Raynor, you and your rebel armies fight not only for the liberation of Terran systems, but also for the restoration of Kerrigan.
This sale will last until Monday, March 11, one day before the release of Heart of the Swarm. So if you haven't picked it up yet, now is as good a time as any.