(This entry is intended to be the quick "you have 30 seconds to explain TCL to someone" speech.)
Tcl is a rapid-prototyping environment. It is most known as a scripting language, but Tcl is much, much, more. Tcl has been adapted to run webservers, robotic manufacturing equipment, even spacecraft. Tcl is bundled with every major Linux Distro, OS X, even Cisco routers. Odds are that you have probably used a Tcl/Tk application and never even knew it. (If you've watched NBC since 1998, you've seen a Tcl app on screen.)
All this power may sound a little daunting to learn. No worries, Tcl is extremely simple to program. There are but 12 rules a coder needs to be familiar with to competently program in it. This power and simplicity makes it an essential tool for many systems.
If all that were not enough, Tcl has a powerful GUI construction tool called Tk. Tcl/Tk quietly runs a lot of day to day utilities under Linux. If you’ve ever compiled the kernel using Xconfig, that’s a Tk app.
Where can I get Tcl/Tk?
With the notable exception of Windows, if your computer can read this web page, it probably has Tcl/Tk installed. To get started programming, go out to a command line and type "wish". Running Windows? Don't have Tcl/Tk? Have an older version? Our friends at ActiveState have friendly bundles of tools for Windows, MacOS X, and Linux. The bundle includes an up-to-date Tcl/Tk interpretor, a lot of popular packages, and most of the tools you would need to get started on your own killer App.
Tcl/Tk is maintained by the Tcl Core Team on SourceForge. You can follow its developments at: http://tcl.sf.net/