Man Pages:
Tk8.4 Commands
Tcl/Tk Developers:
The Tcl'ers Wiki, and especially that Wiki's page on Tkinter
Tcl Developer Exchange
Tcl/Tk SourceForge Project
Tcllib SourceForge Project, which also includes Bwidgets, tklib, and more
"Tcl-URL!"
Other Projects:
TkTreeCtrl SourceForge Project
ActiveState's Tk Modifications
Miscellaneous:
Tcl in Wikipedia
GTK look for tk, easily applied to Tkinter
Tk patches to improve widget look (3D borders, scrollbars and menubuttons) and to add anti-aliasing to canvases using the Cairo library