The first component of ClassyTcl is a dynamically loadable object system for Tcl. A Tcl-only as well as a C implementation is available. It also doesn't require patches to the Tcl core, so keeping up with new releases should be easy.
Performance is very adequate.
In contrast to other Tcl object systems for Tcl, it is not modeled after the object system designed for a completely different language such a C. IMHO, it better follows the Tcl philosophy. The system is simple, but flexible. Classes and objects are dynamic, and can easily be queried, changed and debugged at runtime. The ClassyTcl Builder can be used to graphically create interfaces. It is invoked via the cbuild command in the bin directory.