checkbox

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.

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

Category Command - Category Widget - a part of incr Widgets