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 8.6 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.
Tkinter API Reference
This site hosts a concise reference to the Tkinter Class API. It's been autogenerated based on live introspection of Tkinter widgets, with links to the Tcl/Tk reference material for detailed explanations.
Why not a full, comprehensive Tkinter reference? A complete, updated Tkinter reference, on par (and current) with the Tcl/Tk reference material, is a mammoth project. Many have started it. Few have gotten close to finishing (the best is John Shipman's, see below). Is it really that much? To give some perspective, the C and Python source code that implements Tkinter checks in at around 400kb; think about how many people (and years) have been involved in developing that. The "source" (nroff) for the Tcl/Tk reference documentation is over 950kb.
Shipman Tkinter Reference
John Shipman created a substantial and valuable reference to Tkinter prior to his retirement from New Mexico Tech in 2013. He passed away in 2017. While it hadn’t been updated, it could still be found on infohost.nmt.edu until 2019, when a reorg permanently shut down the server. To ensure this resource remains available, we're hosting a copy on TkDocs.
Important: The last update was in 2013, based on Python 2.7 and Tk 8.5. It is out-of-date and unmaintained. This mirror is entirely unofficial, and no claim is made as to the ownership of the material. For more information, see the discussion on Issue #37149 in the Python bug tracker.