It is still used often in test automation, probably because of Expect. I use it to automate database backups, performing ETL while acting as an intermediary between Oracle's sqlplus and MS's bcp as well as for one-off GUI utilities.

It really is a shame it lost out to Perl and Python. The language is so damn pragmatic. Programmers who enjoy snapping together executables in shell will feel at home with Tcl. It functions like a portable shell. Also, Tcl embodies the philosophy of simplicity in implementation better any other software I've encountered; the C API is simplicity itself. It's actually a great way to organize large programs too, just make the commands do more work with C algorithms and data structures and expose less text-output-chatter to Tcl, in other words, make the commands do "big" things and glue them together in Tcl. It's BSD licensed. It has a surprising number of packages due to it being so trivial to convert existing C libraries to Tcl commands using the dead simple API.

I'm surprised more open source OSs (Linux & BSDs) have chosen Ruby, Python, and Perl as opposed to Tcl. Tcl seems like the more consistent extension of the Unix Way.

There was a surprisingly large "enterprise" crowd using Tcl, as it's pretty great for self-contained environments. You have to generate the binding to your own proprietary legacy interfaces anyway, and lots of situations benefit from proper event handling and easy enough GUIs (esp. canvas graphics). And every enterprise GUI looks bad anyway, so complaints about Tcls' L&F were rare.

I think it missed some important opportunities, and was too focused on GUI stuff. And after all, the Perl/Python/Ruby bindings to Tk weren't that bad, either. In the early days of the web, tcl actually was a pretty good option (as a rival to Perl/CGI). And aolserver was pretty neat (still is, I'd say). But then it didn't go with the times, gtk/Qt stole the Unix OSS GUI thunder, RMS ranted against it, lua came along for embedded interpreters, Ousterhout moved around from Sun, to Scriptics to whoknowswhere… It's easy enough to miss your window.

At least it's still more popular than Pike… I wouldn't rule out a small comeback, there aren't any technical reasons against it, it's well maintained, fast, has a very good codebase etc.