JXTA
JXTA: Secure Java P2P platform with possibilities to share Bandwidth, Storage and Processing power
It's also implemeted in C, C++, Perl, Python. The C implementation is very complete.
See Also:
Here is a very good document to get you into JXTA in a few minuates, well written and complete with links to subprojects (Dec 2002) http://developer.java.sun.com/developer/technicalArticles/Networking/jxta/