RetroShare is a cross-platform, secured and decentralized communication system. It lets you to securely chat, share photos, videos, and more with family and friends, using OpenPGP to authenticate peers and OpenSSL to encrypt all communication.
A cross-platform GUI library
wxWidgets is a C++ library that lets developers create applications for Windows, Mac OS X, Linux and other platforms with a single code base. It has popular language bindings for Python, Perl, Ruby and many other languages, and unlike other cross-platform toolkits, wxWidgets gives applications a truly native look and feel because it uses the platform's native API rather than emulating the GUI. It's also extensive, free, open-source and mature.