Tcl is "in competition" with Perl, Bash, Ksh, and (somewhat) Python. In the PC environment, Tcl/Tk would certainly offer advantages over Visual Basic. Tcl doesn't seem (imho) to compete with Java, which is more like a competitor for C++. In fact, at Sun Tcl is being integrated with Java; Tcl (as the scripting language) and Java (as the system-level compiled language) is a good combination. It's odd, but Tcl might just be the best-kept secret in the computing world: although it's widely used in both academia and industry, and plenty of folks save lots of time and money by using it, somehow it doesn't get much coverage in the trade press. In fact, Tcl/Tk has sometimes been called "the best-kept secret of the software industry."
Q: How many Tcl programmers does it take to change a light bulb?
A: 500,000. But no one knows they're doing it.
What kinds of uses are people making of it? Well, some highlights: The main NBC newsroom is controlled by a realtime system held together with Tcl, with Tk GUI. Sybase owns a million-line test suite written in Tcl. Oracle uses Tcl in their Enterprise Manager product. Oil rigs in the Gulf of Mexico are controlled and monitored using Tcl. The Pathfinder mission used Tcl. ESO (European Southern Observatory) based its flagship image processing interface on Tcl. AltaVista uses Tcl. Check the Proceedings of the annual Usenix Tcl Conference, and you may be surprised at the variety of projects constructed with Tcl and Tk. There's even a Tcl plugin for Netscape.
Here at Lick Observatory, we use Tcl for building GUIs for large instruments (like the ESI Spectrograph), as well as for quality assurance testing, documentation and code generation, and a lot of other applications; see Tcl/2K Conference Poster for some other astronomy applications.
At this site we mostly use Tcl/Tk for two reasons: first, Tk is the fastest environment in which we have ever developed X11 (GUI) applications; second, Tcl is a terrific language for list manipulation (without the high learning curve and weightiness of LISP) and since I focus largely on database applications and "information mechanics," a reliable and quick-to-write language for list processing is very valuable. Tcl is a killer language for database applications -- more details on that aspect in the Guided Tour.
Tcl/Tk is also FREE (given our budgetary situation, this is an important virtue!) and Open Source. The core is supported by Scriptics, and you can get professional support and consulting from them and from other companies; but an enthusiastic international community of hackers supports a large library of extensions to the language. We have little fear, therefore, that it will be "orphaned" as some more obscure languages have in the past. The Tcl/Tk developer community is one of the exemplary Internet communities: friendly, generous, and devilishly ingenious. Check out comp.lang.tcl!
For more (and much more official) information about Tcl, see
The Scriptics Home Page (for news about the core Tcl distribution),
The Tcl/Tk Home Site (the online manpages are especially good!),
and/or The NeoSoft Tcl Page (for the lowdown on Tcl contributors and free extensions).