[What is ActiveX? Where does one read about it?] A good place to start might be: http://www.microsoft.com/com/tech/ActiveX.asp
Tcl extensions that support ActiveX in some way include ActiveTcl, TclScript, Optcl, TclControl, Noel Frankinet's AxTcl (well, not really his), TclBridge, ...
Include tcom here, as ActiveX objects are basically lighter-weight COM objects.