There are several options available for obtaining Tcl/Tk. These are listed below.
Operating System Packages
Most Unix / Linux operating system distributions, as well as Mac OS X, include Tcl/Tk. If not already installed, you can use your system's package manager to install the appropriate packages. For example, on a Debian system, you can type
apt-get install tcl
to install Tcl. Note than some components, even including Tk, man pages and C libraries may be separate packages and installed in a similar manner.
To check the version installed, start Tcl/Tk (usually via running "tclsh" or "wish" from a command line), and typing
This will tell you what version you're running.
If you do not have privileges to install software or if you need a newer version than the one provided by the system, you may use one of the alternative means of obtaining Tcl as described below.
Pre-built Tcl distributions for many common platforms are available for download. These include both traditional installers as well as single file "download and run" executables. See Binary distributions.
Finally, you always have the option of downloading the Tcl/Tk source distributions and building it yourself. See Source distributions for instructions on downloading and compiling.
A very large number of libraries and extensions are available for Tcl/Tk. The most popular and commonly used ones are catalogued in the Great Unified Tcl/Tk Extension Repository. (In other words, get them from the GUTTER.) The Tcler's Wiki is an alternate, but uncategorized, resource for the same.
Tcl/Tk is open source (based on a BSD-style license), so you can
use it and modify it virtually any way you want, including for
commercial uses. Here is the Tcl source distribution
license, but be sure to check the license for any particular
distribution you use.