Here are a few little gems that you might be able to use with Tk widgets.  Other goodies can be had at [Tk Examples] or [Bag of Tk Algorithms].

Note that most of these little tricks are documented in the
man pages.  Read them carefully, and you'll find all kinds
of wonderful little things you can do.

Check out [Tcl Gems] for some other good ideas.

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'''Inserting in a text widget'''

You can programmatically insert text into a text widget using
the ''insert'' subcommand.  You have to specify ''where'' you want
to insert the text with an index, which can be one of several forms.
You can specify the line number and character position with using
the "line.char" format or the special index name ''end''.  You can also do a little math using the incices, like "end - 2 lines"

You can also use ''marks'' to specify locations in the text.  You can
add your own marks, or use one of the special marks.  The mark 
''insert'' indicates the current cursor position, and the mark 
''current'' indicates the current mouse pointer position.  So this 
command will insert new text at the current cursor position:

  .t insert "current" $newText

[Arts and Crafts of Tcl-Tk Programming] - [Useful Tk Widgets]