Android/iOS/Windows/Web: Today, Microsoft announced brand new Android and iOS apps for its Xbox Music service. With the simultaneous addition of the top two mobile platforms, Xbox Music joins the big dogs of music streaming.
You can only use the mobile apps if you have a paid Xbox Music Pass subscription service (a common approach; most streaming services only offer mobile access to paying customers). One potential downside: neither mobile app currently allows you to download music for offline use. Microsoft says that will be added to a future release, but right now that's a gap compared to Spotify or Play Music.
On the upside, Microsoft has opened up its web-based music player to non-subscribers by offering free, ad-supported access to the same library that Windows 8 users can access via the built-in app. The service isn't entirely unlimited (you're restricted to 10 hours a month after the first six months), but that still makes it one of the few free options out there.
Xbox Music [iTunes App Store]
Xbox Music [Google Play Store]