Easiest to Access from Anywhere: Google Music Beta
A few weeks back, Google launched its music site, to much fanfare. The service promises to upload your iTunes collection, store it on its servers (or in the cloud, as tech parlance has it), and then let you stream it to any device. There are downsides: Uploading can take many hours and is capped at 20,000 songs, which is not that much for a true music-ophile. A long-term monthly fee hasn't been announced yet. The service is currently in the Beta product stage and Tom's Guide hasn't yet received access from Google to take it for a hands-on spin.
A potential risk of this service is that the music industry could potentially come knocking and take back songs that were downloaded illegally, meaning your collection could dwindle. Currently Google is only allowing access to Google Music Beta via personal invite.
On the other hand, Google music is easily accessed on the iPhone, using Safari, or on any Android. You can use the Instant Mix feature to automatically build new playlists of songs from your collection that go great together. All the playlists you create and all the changes you make to them are automatically available everywhere your music is. You can upload your personal music collection to a single library, even if it's scattered across multiple computers. And when you add new music to your computer, it can be automatically added to your music collection online. And because it’s Google, your collection will be easily searchable.
You can listen from the web or any enabled device with the Music app available from Android Market, or from Safari. And if you’re not online, the songs you've recently played will automatically be available offline. You can also select the specific albums, artists and playlists you want to have available when you're not connected.