The original operating system for the Apple [L1 ] Macintosh Computer (which was not the first computer from Apple - it was actually a fourth or more generation computer). Also called MacOS Classic to differentiate from MacOS X.
These days, most Macs are coming with MacOS X installed.
Note that Daniel Steffen and others provide Tcl, Tk, and even Tclkit in formats suitable for MacOS Classic (both 68k and Power PC!), up to Tcl/Tk release 8.4.2.
However, support for the MacOS Classic platform was dropped from Tcl/Tk as of version 8.5.
DKF vaguely remembers the Apple ][, which is kind-of awkward to type in the Wiki... LV has several models of these still... a II, IIgs and probably a IIe somewhere in the house. I don't think anyone ever successfully got Tcl running on an Apple II though.
See QuickTimeTcl as a Mac related extensions (also available for Windows).
The archive for the main MacOS Tcl mailing list appears at http://sourceforge.net/mailarchive/forum.php?forum=tcl-mac
Daniel announces release of MacOS X 8.4.5 binaries [L2 ] of his TclTkAquaBI distribution. Note particularly the list of all the neat extensions he builds in (or doesn't, depending on which flavor of his binary you choose).