aMSN

aMSN (Former known as Alvaro's Messenger) is a client for the "MSN Messenger" instant messaging network, and is written in Tcl/Tk.
Probably one of the most popular Tcl/Tk applications out there.
Interview with Alvaro J. Iradier, AMSN Founder and Lead Developer [1]

aMSN features:

Display pictures
Custom emoticons
Multi-language support (around 40 languages currently supported)
Webcam support
Sign in to more than one account at once
Full-speed File transfers
Group support
Normal, and animated emoticons with sounds
Chat logs
Timestamping
Event alarms
Conferencing support
Tabbed chat windows

 What: aMSN
 Where: http://www.amsn-project.net/
 Description: Multi-lingual Tcl client for MSN Messenger
        Currently at version 0.97.2
 Updated: 07/2008
 Contact: See web site
 Portable version: http://portableapps.com/apps/internet/amsn_portable

aMSN is a fork of Compu's MSN Messenger (ccmsn), a project that is no longer active.

 What: Compu's MSN Messenger (ccmsn)
 Where: http://msn.CompuCreations.com/
 Description: A Tcl/Tk based MicroSoft Network Messenger chat client.
        Currently at version 0.24 .
 Updated: 09/2001
 Contact: mailto:dave@CompuCreations.com

X Messenger

http://sun.systemnews.com/articles/118/2/OpenSolaris/19046

mentions that in 2007, Open Solaris support for this program was seen.