I don’t know what the difference is between downloading and sharing libraries, but check out Soulseek. It’s a little file-sharing program that lets others download directly from your music folder and vice versa. When I couldn’t find an album on any torrent tracker, some hero on Soulseek had it. The main drawback compared to torrents is that you’re limited to the upload speed of this one person, you can’t connect to a dozen people with the same files and combine their power.