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 {}

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

        exit 

----

[Category TK]