TeamWave developed a variety of collaboration tools, using 
Tcl and Tk extensively.

The standalone product, TeamWave Workplace, features a Tk-based
desktop application offering whiteboard, chat, and a variety of
other collaborative tools.

The newer web based versions don't require users to download 
client software.
Though Tk is not used for the GUI (it is Java-applet based, again
so that no download is required), Tcl and a Tk-like API are used
on the server to drive the user interface.

The various products are written in a mix of Tcl and C, and rely
on MetaKit for storage.  The web versions also use AOLserver.

For more information, contact [Mark Roseman].

The original web site [http://www.teamwave.com] has been essentially
shut down, but Mark maintains a retrospective site
[http://www.markroseman.com/teamwave/] with some additional links
and information.