gtk-vnc is a VNC viewer widget for GTK.
It provides a core library written in C, using coroutines to allow it to be completely asynchronous while remaining single threaded. A wrapper library using PyGTK provides access to the widget functionality from Python. Two example programs illustrate use of the widget by re-implementing the traditional 'vncviewer' in a few 10's of lines of code.
The client is fully IPv6 aware, and as well as the traditional VNC authentication protocol, implements the VeNCrypt extension to provide secure TLS/SSL encrypted communications, optionally using x509 certificates to authenticate.
Some applications using GTK VNC include
Vinagre - John Wendell's general purpose VNC client application for the GNOME project
Virtual Machine Manager - GUI for managing Xen, QEMU and KVM virtual machines.
KVM Test - Virtualized OS install automation with KVM.