For a quick overview of Tk, and in particular where it stands today, see the Tk backgrounder page.
Languages using Tk
For brief information and links to some of the languages that have Tk bindings available, see the languages using Tk page.
Official Tk Command Reference
You can find a copy of the official Tk 8.5 command reference documentation at the main Tcl/Tk developer site. Authored by the developers working directly on the Tk core, this is the same man-page style documentation you'd get if you downloaded the official Tk distribution. While this is obviously Tcl-oriented, it provides the most complete reference to the individual behavior of Tk widgets.