Windows/Mac/Linux: Despite the rising popularity of BitTorrent, for many people there's still just one name in file sharing: LimeWire. The widely used LimeWire actually does support BitTorrent these days, and yesterday, LimeWire released an alpha version of the new and improved LimeWire 5, adding private file sharing features that will allow user to privately share folders with friends. As Wired is quick to point out, LimeWire 5 still supports the downloading from strangers features that made LimeWire popular, but the new default settings publicly shares only files you downloaded from the public network. You can specifically set permissions for all the rest, including which files you want to share and with whom you'd like to share them. How does it know who your private contacts are? Through your Gmail account, of course!
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Actually, as of this writing, LimeWire 5 supports Gmail (along with any other Jabber account) and LiveJournal. Once you log in with your Gmail account, you'll see your friends in your sidebar. According to Wired, LimeWire is looking into importing friends from Facebook and other social sites. You can chat with any of your friends from inside LimeWire 5, tell them to go download and install the alpha, and then it's sharing time.
Once you've found a friend you want to share with, just pick the files you want to share and go nuts (assuming your friend has installed LimeWire 5). Head over to Wired's review for an extremely detailed rundown, or download the alpha and start sharing with your friends. LimeWire 5 Alpha is a free download for Windows, Mac, and Linux. It's an alpha release, so you should expect some bugs, but the homepage claims that the core LimeWire functionality is a-okay.