TeamWave developed a variety of collaboration tools, using Tcl and Tk extensively.
The standalone product, TeamWave Workplace, featured a Tk-based desktop application offering whiteboard, chat, and a variety of other collaborative tools.
The later web based versions didn't require users to download client software. Though Tk was not used for the GUI (it was Java-applet based, again so that no download is required), Tcl and a Tk-like API were used on the server to drive the user interface.
The various products were written in a mix of Tcl and C, and relied on MetaKit for storage. The web versions also used AOLserver.
For more information, contact Mark Roseman.