Green Man Gaming 25% off everything voucher for multiple orders until August 10th according to a post on their blog today. Here's the link:
I've been using the daily 25% off deals for the last couple of days to pre-purchase Darksiders 2 and Dark Souls. I paid about $38 and $30 respectively. Also they work on steam. Though the game page for the game you want has to specify that Steam is required for it to do so.