The Zune has wireless song sharing built in, but it comes with a three-playback restriction and--let's face it--most of the world has iPods. No worries: iPod sharing is coming in the form of a $100 gadget available later this month.
The concept's simple: connect miShare to two iPods (it doesn't work with first- or second-generation iPods, nor with the Shuffle or Touch). Then set the switch to music, video, or photo, press the miShare's button, and it'll transfer the last-played audio or video file, or all the photos in a folder you pre-define, from the home iPod to the receiving one. You can also transfer entire On-The-Go playlists between the devices by holding the transfer button down for three seconds.
Minor drawbacks: getting the songs from the iPod back to your computer will be difficult unless you use a third-party add-on like MusicRescue. The transfer rate's about 500 KB per second, which means an album will take a couple minutes, and the device doesn't offer any trickery to get around Apple's FairPlay DRM scheme, meaning that most files bought from iTunes won't be playable after transfer.
Will you get busted for using miShare? It doesn't have any provisions to get around DRM, so it wouldn't seem to violate the DMCA. And it's certainly legal to transfer files of songs you own the copyright to. (One of my big complaints about the Zune wireless sharing feature is that even if I transfer music from my own bands to you, it still expires after three plays.) But using miShare to transfer large quantities of music seems to exist in that same grey area as copying songs to a flash drive to give to a friend. Probably not strictly legal, but almost impossible to enforce, and probably not worth copyright owners' time--if they're going to sue customers, they're going to try for the ones who are making thousands of files available to thousands of users simultaneously through file-trading networks.