Here are some web addresses that will provide you with everything from source code to complete applications, and everything in between:
Scriptics Corporation, the current maintainers of Tcl/Tk:
http://www.scriptics.com/
The Official Tcl/Tk Contributed Sources Archive:
http://www.neosoft.com/tcl/default.html
An organization dedicated to promoting Tcl/Tk:
http://www.tclconsortium.org/
An extensive resource for Tcl/Tk information:
http://www.tcltk.com/
Home of Don Libes' cgi.tcl library:
http://expect.nist.gov/cgi.tcl/
Steve Ball's article on Tcl HTTPD:
http://www.zveno.com/zm.cgi/in-features/199802-tclhttpd.html
Brent Welch's white paper on Tcl Web Servers:
http://sunscript.sun.com/tclhttpd/server.html
Active newsgroup for everything about Tcl/Tk: comp.lang.tcl
Whether you're using Red Hat, Slackware, Caldera, Debian, S.u.S.E. or MkLinux, you'll find a copy of Tcl/Tk along with all the other packages, although the version you have depends on how recent your distribution is. As of August 1998, the latest version is 8.0.3.
If you'd prefer to download the source or precompiled binaries from the Net, the place to start is http://www.scriptics.com/. This site contains all the archives you need to run the latest version, along with copies or pointers to copies of popular Tcl extensions.