D-Bus is an interprocess communication system via a message bus, a simple way for applications to talk to one another. For more information, see http://www.freedesktop.org/wiki/Software/dbus
There are currently two projects for providing DBus bindings for Tcl:
xanguli : xanguli ain't no gui library - leverages the tk toolkit from tcl via dbus. This allows the use of Tk from any programming language that has dbus bindings.