Last week Valve released the Steam app beta into the wild. Within hours hundreds of Steam users were receiving messages through Steam chat on their PCs saying things like "OMG I'm sending this from my mobile can u tell?" except with more spelling mistakes, because their fingers were shaking with raw, on-the-go excitement.
It's now freely available to everyone with an iPhone or Android device on the App Store and Android Market respectively. It'll let you access Steam chat and buy games through Steam on the fly. A vital tool for Steam sales to come.