It's traditional--perhaps dysfunctionally so--for Tk-ers to code their own progressbars, often on the model found in the "Bag of Tk algorithms". It's certainly true that Tk makes it easy. mkWidgets and BWidgets build in progressbar widgets.
Also a nice and simple progress bar is the one by DKF at: http://www.cs.man.ac.uk/~fellowsd/tcl/mwidx.html#progress It is also able to display the percentage along with the filled rectangular...
George