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).
These days, most Macs are coming with MacOS X installed.
Note however that Daniel Steffen and others continue to provide Tcl, Tk, and even Tclkit in formats suitable for MacOS (both 68k and Power PC!).
DKF vaguely remembers the Apple ][, which is kind-of awkward to type in the Wiki... LV has several models of these still - though that is likely to change in the upcoming weeks.
See QuickTimeTcl as one of the Mac specific extensions.