how about a 'Gaming Bargains/offers' sub-forum where people can make new threads about steam offers, D2D offers and general retail offers from shops/sites? I'd even go as far to say make it multi-platform. There's plenty of supermarket console offers that need to be shared. I think a sub-forum dedicated to just Steam is maybe going a bit far, It wouldn't be much quicker going through that than just spending 5 minutes on the Steam site. A gaming bargains/offers thread would show you offers in places that you would otherwise miss.
At the moment we only have 2 games sub-forums, PC games and Console games. If we had a 3rd Gaming bargains then it would move all of the buying threads out of the other 2 and into the new sub-forum leaving the games sub-forums for just that, talking about games.