Gazzera is a secure, friend to friend file sharing application. It uses many open source packages such as Gtk2, Glade, Sqlite, Openssl, pcre, etc. It's runs on windows and linux (should be easily portable to mac).
It's developed with FPC and Lazarus, the sources are avaiable under GPL.
http://gazzera.sf.nethttp://sourceforge.net/projects/gazzera