From the XOTcl homepage:

Extended Object Tcl (for short: XOTcl, pronounced exotickle) is an object-
oriented scripting language based on Tcl. It was originally designed for 
providing language support for design patterns and provides novel constructs 
such as filters or transitive mixin classes. The language is designed for 
empowering rather than constraining system developers. The basic object model 
is highly influenced by CLOS.

XOTcl is contained in the Industry-standard Tcl distribution ActiveTcl, 
which is available for several platforms, it is part of the Batteries Included 
Distribution for Mac OS X and it is shipped as part of the developer support 
for Mac OS X Tiger. 

WWW: http://media.wu-wien.ac.at/