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.