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Spotify automatically connects with your iTunes, your Windows Media Player and connects your offline music to your playlists, too. It's all consolidated in Spotify, allowing you to transfer music into it to play it from one window.
In Offline mode (a subscription is needed), you can keep your streamed tracks locally so there will be no cut-off in music -- even if your web connection fails you!
For more on Spotify, read the corresponding post on the ZDNet iGeneration column.