Using this page to "crowd-source" some text for http://www.tcl.tk/software/tcltk/8.6.html .
The commands of the TclOO package are now part of Tcl itself. This gives Tcl a built-in object system that is fully dynamic, class-based, and includes advanced features such as meta-classes, filters, and mixins.
New version 4 of the popular package Itcl (aka incr Tcl) is also included, granting support for some traditional OO Tcl programming out of the box as well.