There are checkbox widgets in serveral widget sets.
You might also have been looking for checkbutton widget.
Tile includes a checkbox widget.
For instance, see 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