If your pocket telephone has a Halo lady living inside it, good news: Valve have released a Steam app for Windows Phone. Which is mostly exciting because it means Windows Phone now has the Steam mobile authenticator, so you can more easily secure your account with two-factor authentication and also get past those holds on trading for unauthenticated users.
The app is this-a-way and supports Windows Phone 8.1 and above. It is labelled as only the initial release with "limited functionality" but hey, it's better than a punch in the face. Many things are, really.
It looks like Valve have brought in the chap who'd made an unofficial Steam authenticator for Windows Phone.
Say hi to Cortana for me!