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I'm sorry, I can't help it - I'm a sucker for mashups. Which is why I have to bring the latest one to your attention: Tumbltape.
It's exactly how it sounds: a Muxtape for Tumblr. Just when you thought Muxtape was gone for good, it has inspired John Zanussi and Adam Gotterer to catch on the music sharing wave and create Tumbltape, which takes the latest songs (aka - audio files) from each user's tumblelog and creates a playlist.
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When new songs are added, your Tumbltape playlist is automatically updated. Plus, you can choose to let Tumbltape figure out the artist and track names based on the text from your post, or you can type in the artists and track titles yourself. Simple as that.