Way back in 2003, there was a brief window (between iTunes 4.0 and 4.0.1) where iTunes music could be streamed willy-nilly over the Internet; as long as you had the IP address of the source machine, you could play tracks from all across the globe with no difficulty, rather than being limited to the other iTunes users on your local network. That bucolic interlude (also known to Apple's legal team as "the great unpleasantness") was shortlived, and subsequent attempts to broaden the local-subnet limitation on iTunes sharing have been equally transient or otherwise undesirable (Windows only, costly, over-social or too tricky for casual use). [For clarity, what iTunes calls 'sharing' is technically streaming; the remote tracks are only accessible while the source machine is online, and you cannot copy shared music to your local library, at least not easily/legally.]
Now, Simplify Media has released a free beta of its namesake music buddy tool, designed to expand the Shared list in iTunes to include up to 30 of your friends' music libraries, regardless of physical proximity. Using the SM app is as simple as advertised: download for Mac or XP, sign up to create your sharing name, and invite friends to become your buddies. After the SM app scans your music folder (took a while for me), just leave it running; your buddies' music libraries show up in your iTunes shared list, and yours shows up in theirs. Playback and browsing was quick and stutter-free on my DSL connection, and likewise for the remote users browsing my library.
There are a few caveats with this beta: other music player apps are unsupported, as is iTunes on Windows Vista;
there is a 'Buy Music' ad window which pops up annoyingly and can't be closed (although you can minimize it to the Dock); (the 'Buy Music' window has been removed from the latest build) and of course there aren't that many people using it yet, so who to share with? Other than that, my quick evaluation is that Simplify Media does the job: it gives you access to your friends' music while staying (mostly) out of your way.