Android (2.3+): If the cloud is where you store your music, then CloudAmpz for Android is a great way to listen to it on-the-go without downloading those files. The app works with Google Drive, Box and Dropbox, but only one account at a time.
In many ways, CloudAmpz is similar to Beat for Android, but it has a few options that make it worthwhile. CloudAmpz does a great job of showing your current playing track's lyrics by using the ChartLyrics database. Much like Beat, it will also fetch album art.
The other plus is in performance. CloudAmpz is better at streaming high-bitrate files than Beat or any other such player -- the playback is smooth. It also has an equaliser, lets you set up custom playlists, and supports playback local storage.
The free version of CloudAmpz includes ads -- if you are going to use it to make the most of your cloud storage, pay a dollar for the Pro version. Playing back FLAC files while loading internet-based ads can strain your connection.