On my Android Phone, I use Google Play Music. I install an app on my computer that syncs my music with Google's cloud. On my phone, I can listen to this music whenever it is connected to the internet.
It would be nice to do something similar on Windows Phone. My music library is far too large to fit on my phone, so a streaming service is necessary.
You can utilize DLNA to access media on other devices in your home network (this is how most devices pull this off). This link will bring you to a detailed article on how to set it up.
If you have a Nokia device you have access to an app called "Play to", you will find in your "Nokia Collection" marketplace option.
htc has this option as well under their HTC marketplace option, the app is named "HTC Connected Media"
Now if you are looking to do this from the cloud, you can place the music in your SkyDrive and stream it anywhere.
