Tcl 7.4b3 and Tk 4.0b3 are available via ftp at ftp.cs.berkeley.edu:/ucb/tcl/ as tcl7.4b3.tar.Z and tk4.0b3.tar.Z (see release information).
John Ousterhout's book and the latest version of Tcl and Tk can be found at ftp://ftp.cs.berkeley.edu/ucb/tcl . John Ousterhout's book is also available from CICA, Indiana University.
Another book available online is Brent Welch's Practical Programming in Tcl and Tk (~700Kb for the gzip-ed Postscript version), to be soon published by Prentice Hall.
A nice hypertext version of the FAQ can be found at http://www.cis.ohio-state.edu/hypertext/faq/usenet/tcl-faq/top.html and is mirrored (with online Man pages and other goodies) at http://www.sco.com/Technology/tcl/Tcl.html .
Mike Hopkirk's TCL WWW Info has very extensive info about Tcl (check its Tcl/TK Resources entry (alternate path to these resources), under the Other Tcl Info Collations heading). Mike Hopkirk's TCL WWW Info page is mirrored at http://www.x.co.uk/of_interest/tcl/Tcl.html, http://www.isu.edu/tcl/Tcl.html and http://ita.tutkie.tut.ac.jp/tcl/Tcl.html
Online help is available again from Fermilab, at http://www-sdss.fnal.gov:8000/ftcl/extended/tcllib/help.
Have a peek at John Ousterhout's home page and Release Information for Tk 4.0b3 and Tcl 7.4b3
Nat Pryce has put up an Index to Itcl Widget Documentation
Kevin B. Kenny has an interesting document analyzing Dynamic Loading for Tcl .
David Martland has written an intoduction to Tcl/Tk. This document is still under development.
Wayne Christopher has collected some News articles comparing Tcl with other systems.
The Web of Wonder has an entry for Tcl/Tk
For those interested in turning character-based interactive programs into batch ones, have a look at Expect (there is an alternate site, in France).
TIX is a set of compound widgets, including a Combobox, a Motif style FileSelectbox, a PanedWindow widget, a Notebook widget and many more, built on top of TK. It is written completely in TCL and requires no extension to the C source of TK. I personally like its pop-up menu facility. The author of TIX has his Home Page on the web.
Pacco is a set of widgets that extends the toolkit Tk to supply object visualization.
A Prolog interface to the Tcl/Tk toolkit is available from the "European Computer-Industry Research Centre" (ECRC), in Germany.
Object Tcl is a Tcl extension package that makes it possible to use object oriented programming concepts in Tcl and use/reuse C++ classes.
A portable interface for Tk (PortTk) has been developed by Filip Karlemo (fkarlemo@snakemail.hut.fi), Ari Kiviluoma (akiviluo@snakemail.hut.fi) and Kimmo Joki-Korpela (kjk@snakemail.hut.fi) as a project course at Helsinki University of Technology. (There is an alternate path to this information)
Tk for Windows is available from ftp://ftp.ms.uky.edu/pub/tk-win/ (University of Kentucky). Look at their FAQ file for more info. Mirror sites are: ftp://ftp.ibp.fr/pub/tcl/tk-win/, ftp://mrrl.lut.ac.uk/tcl-tk/tk-win/ and ftp://sunsite.unc.edu/pub/languages/tcl/tk-win/
Another Tk for Windows is available from ftp://ftp.sra.co.jp/pub/lang/tcl (Software Research Associates, Inc., in Japan), in files tkwine*. "exec" and "send" are not supported! See file twinebin.doc for a list of characteristics and limitations.
Tcl 7.3, TclX7.3a, Tk3.6 for MSDOS & Desqview/X can be found at ftp://harbor.ecn.purdue.edu/pub/tcl/distrib/, in files mstcl73s.zip (sources) and mstcl73e.zip (executables). They are mirrored at ftp://ftp.germany.eu.net/pub/programming/tools/tcl/distrib/.
MacTcl 7.3 (partial port to the Macintosh) can be found at ftp://ftp.germany.eu.net/pub/programming/tools/tcl/distrib/ in MacTcl7.3.sea.hqx (see the README file).
Liem Bahneman <roland@cac.washington.edu> has announced tkHTML-2.0 , a tcl/tk-based HTML document editor for X11. It has almost all the standard editing features of a word processor, as well as special functions for editing HTML documents. TkHTML also has WYSIWYG previewing.
Jay Sekora has put in the public domain a set of tools (text editor, directory browser, hypertext help-file viewer, etc. ) named jstools, archived on ftp://ftp.aud.alcatel.com/tcl/code/