DGAF is a very new peer-to-peer communications program.
DGAF networks are private [to some extent], Master Nodes may password protect their networks.
How Much Does It Cost?
100% free, but any donations would be gladly accepted. Most of project is released under a BSD license (even if it says otherwise), however some of the project is under LGPL.
Aren't there enough file sharing programs out there?
This used to be the case; However because of certain greedy entities, sharing publicly is not such a good idea, instead smaller networks where peers are familiar with each other in some way, is seemingly the future of P2P networks.
What Operating System Will It Run On?
It is known to compile/run on Windows, Linux, Mac, Solaris, and FreeBSD. However it should work on any POSIX OS with GTK+ 2 and a C Compiler.