Windows and Mac OS X: If you use iTunes and have multiple machines, you've been confronted with the difficult problem of keeping your libraries in sync. MediaRover solves this problem both well and for free.
There are a number of (sometimes costly) iTunes syncing solutions, but MediaRover is both free and pretty amazing. While MediaRover is billed as sync for the entire family, it works all the same if you've just got multiple machines yourself. If you add a song to one computer, other networked computers get it as well. It'll even back up to your NAS. Beyond backup, this is great for streaming to your Sonos, XBox 360, PS3 or really anything that's capable of pulling the content. In addition to syncing music, it pushes metadata updates as well. Any changes you make in one library will sync right up with all your other libraries.
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MediaRover is compatible with both Windows PCs and Macs and is available right now for free.