http://www.freeprogrammingresources.com/tcl.html

(a large list of helpful resources)

http://www.tcl.tk/man/

(Tcl/Tk manual pages)

http://www.mit.edu/afs/sipb/user/golem/doc/tcltk-iap2000/TclTk1.html

(a tutorial describing many commands by describing their implementation in a short program)

http://www.beedub.com/book/

(some sample chapters from Practical Programming in Tcl and Tk, by Welch and Jones)

http://philip.greenspun.com/tcl/

(not the most readable tutorial IMHO, but it does have Tickle-me-Elmo ;) It can be accessed from the link below as well.)

http://www.tcl.tk/scripting/

http://hegel.ittc.ukans.edu/topics/tcltk/tutorial-noplugin/index.html

(a short tutorial on the essential Tcl commands, also includes a manual of Tcl/Tk commands at the website below)

http://hegel.ittc.ukans.edu/topics/linux/man-pages/index/index-mann.html

http://pages.cpsc.ucalgary.ca/~saul/personal/archives/Tcl-Tk_stuff/tcl_examples/

(Tk widgets with screenshots)