Use a free cloud music storage service to store your music library online and stream it from all your devices. I've collected many gigabytes of digital music from ripped CDs, iTunes, and other online music stores over the years. It's great to be able to access my entire music library from all of my devices anywhere with an internet connection. My collection is also backed up in case anything happens to my hard drive or other backups. Most of the big music stores include cloud music storage, but some do it better and for free.
Free cloud music storage from Google Play
Free cloud music storage with Google Play Music
Play Music is Google's answer to iTunes. It includes an online music store and streaming service, plus free cloud music storage for up to 50,000 songs, regardless of where they were purchased. Play Music purchases are automatically stored online for streaming, and old purchases can be uploaded from a computer through the Music Manager application.
With the free cloud music storage from Play Music, your music library can be streamed, downloaded, and managed from the Google Play Music app for iPhone and iPad, Android, or on a computer web browser. Get more info and see the steps required to get free cloud music storage with Google Play Music.
Limited online music storage with Amazon
Amazon also automatically stores purchases from the Amazon mp3 Store online, and up to 250 songs from other stores may be imported for free. Upgrade cloud music storage to 250,000 songs for $25 per year. Access songs on the Amazon Music mobile app for iPhone and iPad, Android, Amazon, or other devices. The Amazon Music app also provide access to free music streaming for Prime members. Get more info on uploading your music library to Amazon here.
Paid music storage with iTunes
Apple has no free online music storage for songs purchased outside of iTunes. They do have iTunes Match, a $25 per year cloud music storage service. iTunes Match doesn't require uploading your music library, instead it scans your music library for songs that are in the iTunes Store, and uploads any that it doesn't find. Songs that are already in iTunes can then be streamed back in the 256kbps AAC formation, regardless of what format they were stored at on your computer. Listening to iTunes Match songs is built right into Apple devices. iTunes Match also gets you ad-free streaming on iTunes Radio.
Wrap-up
Google provides a great free cloud music storage and streaming service. It has several nice playback features, such as the ability to create instant mixes (Pandora stations from your library) or custom playlists. I recommend using it to back up your music library, even if you don't plan to stream songs through it. 50,000 songs should be enough to back up most people's full music libraries. If you switch from iPhone to Android or vice versa your music library will be there waiting for you.