Although standard [Tk] doesn't supply a toggle button, and most developers seem
content to use a [checkbox] in its place, 
[Uwe Klein] illustrates how easy it is to customize a [button] by refinement of
a [label]:
    set ::state OFF
    label .lb -textvariable ::state -relief raised -bd 4
    bind .lb <ButtonPress-1> {
        if {"$::state" == "OFF"} {
            set ::state ON
            %W configure -relief sunken
        } else {
            set ::state OFF
            %W configure -relief raised
        }
    }
    label .l -textvariable ::state
    pack .lb .l -side left

[[Comments:  all buttons thus; deserves abstraction; backward-compatible only to 8.?]]