There are checkbox widgets in serveral widget sets.

You might also have been looking for [checkbutton] widget.

[Tile] includes a [checkbox] widget. And it appears that, in [Tk] 8.5, a [Ttk] checkbox will be available [[can someone add a screen image here?]]

Another extension which includes a checkbox is [iwidgets].

[http://incrtcl.sourceforge.net/iwidgets/iwidgets/checkbox.gif]

Docs can be found at http://incrtcl.sourceforge.net/iwidgets/iwidgets/checkbox.html and http://purl.org/tcl/home/man/iwidgets3.0/checkbox.n.html

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[Category Command] - [Category Widget] - a part of [incr Widgets]