Earlier today Frozenbyte (Trine, Splot, Shadowgrounds, etc) announced their Summer Seal. They are giving away thousands of Steam keys and coupons for Shadowgrounds, Shadowgrounds Survivor and Trine, as well as Trine 2.
Unfortunately, the giveaway was more popular than they thought and the Summer Seal page has a message that says it will be back within a day or two. I would just bookmark the page or follow Frozenbyte on Facebook/Twitter/RSS so you get a better chance at getting a free game/coupon.
These are the prizes/coupons they are giving away.