Probably the most common way people interact with Tcl is through python, ironically. Tcl has a GUI library called Tk, through tkinter, available as part of the python standard library.
Outside of Tk, Tcl pops up in a couple of odd places, usually as part of some testing system (expect(1) and SQLite use it) or build system, though use has fallen off quite a bit since the 1990s.
Placing somewhere between a lisp and shell, it's incredibly effective as a language for gluing things together and creating DSLs, and is fairly easy to embed in a manner similar to Lua.
Definitely not a perfect language, but one that I find extremely comfortable to work in and iterate quickly for certain projects.
Antirez's blog has a pretty concise explanation of Tcl's features and what makes it special: