GameStop has sold digital content for ages, or at least the funds to unlock said digital content. One thing was always missing on that disheveled rack of PC games buried between the Gamecube and PSP games though: Steam.
As 0f right now, you can add credits to your Steam account, sans credit card, at any GameStop location. Just to get it out of the way, no, you cannot trade them back in when your done, immediately eliminating the need for 90% of the comments on this story. Cards come in the $20 and $50 variety, and cannot carry custom amounts. Around 4,400 GameStop locations will be stocking the cards, so chances are, your local branch will have what you need.
