Remember that weekend deal on Steam where every Sonic game was 75% off? Well, that deal is back as a daily deal during the Steam Summer Getaway Sale. Games on Steam include Sonic & All-Stars Racing Transformed, Sonic Generations, Sonic CD and fan-favorite (sigh) Sonic Adventure 2. There’s also a collection with all Sonic games on Steam which is still the cheaper option, though it hasn’t been priced off. Click here for the sale.
Besides this, SEGA also added Steam Trading Cards to Sonic & All-Stars Racing Transformed, which you can get by either playing the game, trading them or by buying them on the Steam Community Market. Valve has put up this handy page that tells you how these trading card things work.