Extending Tcl
A couple of ideas that Tcl pioneered were:<p>(I) design for size, ease of extensibiliy and embedding, and (II) doing threads right.<p>(I) became a feature offered by many other languages, but I think Tcl was the one that popularized it. This crown belongs to Lua now.<p>(II) was not adopted as much as it should have been. I am looking at you Python. Interpreter as a library with interpreter state fully contained in an object, no globals or statics -- simplifies threads immensely. Run different interpreters in different threads, no need for global lock, no need for serialization-deserialization to talk to other interpreters in the same address space.