There's also wxGlade (and some other WxWidgets based designer which I
forget offhand).

OT: I heard that Tk was supposed to be using native widgets like
WxWidgets since like version 8 or something. Well, % info patchlevel
shows I'm using Tcl/Tk 8.4.13; but the widgets still look like regular
old Tk widgets. What am I doing wrong here?

Regards,
Jordan