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GMG has a survey up that comes with a 10% off Voucher code reward. You have until the end of the month to complete the survey. You can use the voucher for anything in the store except for Guild Wars 2.
Use the voucher code CIVGO-DSAND-KINGS to get 25% off Civilization V: Gods and Kings. It makes the game $22.95. It uses Steamworks, so Steam activation is required.