Here is a script to display each widget available in Tk. 
It made the assumption that all widgets begin with a lowercase letter.
 
 #
 # the dynamic list of Tk widgets
 # 
 
 puts " Tk version [package require Tk]"
 puts " ----"
 
 set tkCommands [lsort [info commands "::tk::\[a-z\]*"]] 

 foreach cmd $tkCommands { 
         puts [string range $cmd 6 end]
 }
 
 exit 
It gives : 

 Tk version 8.5.7
 ----
 button
 canvas
 checkbutton
 entry
 frame
 label
 labelframe
 listbox
 mc
 mcmax
 mcmaxamp
 menubutton
 message
 panedwindow
 radiobutton
 scale
 scrollbar
 spinbox
 text
 toplevel

----
It works only with version 8.5 so far.
----
[JH] This completely misses the themed Tk widget set (::ttk::*).
----

[LGT] This script tries to correct this : 

        #
        # the dynamic list of Tk widgets
        #

        puts " Tk version [package require Tk]"
        puts " ----"

        set tkCommands [lsort [info commands "::tk::\[a-z\]*"]]
        set ttkCommands [lsort [info commands "::ttk::\[a-z\]*"]]

        set allCommands [concat $tkCommands $ttkCommands]

        foreach cmd $allCommands {
                puts [string range $cmd 2 end]
        }

        exit 

(The assumption seems not correct since for instance ttk::deprecated is not a widget at all) 

It gives : 

         Tk version 8.5.7
         ----
        tk::button
        tk::canvas
        tk::checkbutton
        tk::entry
        tk::frame
        tk::label
        tk::labelframe
        tk::listbox
        tk::mc
        tk::mcmax
        tk::mcmaxamp
        tk::menubutton
        tk::message
        tk::panedwindow
        tk::radiobutton
        tk::scale
        tk::scrollbar
        tk::spinbox
        tk::text
        tk::toplevel
        ttk::button
        ttk::checkbutton
        ttk::clickToFocus
        ttk::combobox
        ttk::copyBindings
        ttk::cursor
        ttk::deprecated
        ttk::deprecated'warning
        ttk::do'deprecate
        ttk::entry
        ttk::focusFirst
        ttk::frame
        ttk::globalGrab
        ttk::grabWindow
        ttk::label
        ttk::labelframe
        ttk::menubutton
        ttk::notebook
        ttk::paned
        ttk::panedwindow
        ttk::progressbar
        ttk::radiobutton
        ttk::releaseGrab
        ttk::scale
        ttk::scrollbar
        ttk::separator
        ttk::setCursor
        ttk::setTheme
        ttk::sizegrip
        ttk::style
        ttk::takefocus
        ttk::takesFocus
        ttk::themes
        ttk::traverseTo
        ttk::treeview
----

[Category TK]