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