Klystrack is free AND open source (an AHX style tracker)
Sunvox is free for all desktop platforms (a buzz style tracker)
Renoise is actually free for everything apart from rendering audio to disk, but you know this already File sharing has been encouraged more lately (instruments, songs, themes, fx chains). Of course if you use VSTs that complicates sharing.
ShcismTracker is a free implementation of Impulse Tracker
All of these are multiplatform I think.
There is no standard file format to be shared though.