If you're an iPhone owner, chances are your device doubles as your MP3 player. However, if you're the type to bring your entire library of tunes on the go, your iPhone won't have space for anything else. This solution is easily remedied, as Apple's iTunes is more tightly integrated with the cloud than ever.
As long as you're online, all your iTunes purchases (including videos) will be playable regardless of whether or not they're stored on your device. You can even take things one step further by subscribing to iTunes Match and making your entire Mac or PC music collection accessible from the cloud. With these options, you can delete all your songs, save for a few favorites that you absolutely need handy for offline listening.
More: How to Use iTunes Match