Lua [http://www.lua.org] is one of the many languages with a [Tk] binding. A vaguely [C]-styled [OO] [scripting] language developed "half-open source" at a Brazilian institution, characterized by using hash tables as central data structure, even for vectors and lists.  Lua's kernel is unusually compact.  Lua style emphasizes [metaprogramming].

Roberto published the Lua book [http://www.inf.puc-rio.br/~roberto/book/] early in 2004.  Lua has its own 
[Wiki] [http://lua-users.org/wiki].

Lux [http://www.equi4.com/lux] is [Jean-Claude Wippler]'s
experiment at binding Lua together with more traditional scripting
languages ...

Max Ischenko reports success using [SWIG] to wrap Lua libraries
as loadable .so-s exposed through Tcl.  He even has an [incr Tcl]
class he calls "LuaInterpreter".

[mfi]: Yup, that was pretty simple. To compile a .so you just do:
 swig -tcl8 -module lua Lua.i
 gcc -c -fpic lua_wrap.c
 gcc -shared lua_wrap.o -o lua.so -llua -llualib
And the abovementioned itcl class is a little more complex than this sketch:
 
 itcl::class LuaInterpreter {
        public {
                method doFile fname { return [lua_dofile $L $fname] }
                method doString buf { return [lua_dostring $L $buf] }
                method call {func {arg ""}} {}
 
                method popLast {} {
                        set value [lua_tostring $L -1]
                        lua_remove $L -1
                        return $value
                }
                method printStack {} {}
        }
        constructor {} {}
        destructor { lua_close $L }
        common call_template "return %s(%s);"
 }
 
 itcl::body LuaInterpreter::constructor {} {
        set L [lua_open 1024]
        L doString "function _ERRORMESSAGE(msg) print(msg) end"
 }
 
 itcl::body LuaInterpreter::call {func {arg ""}} {
        set op [format $call_template $func $arg]
        doString $op
 }

I found the interface of doString, call and popLast quite sufficient for my needs. Notice also, that you can have as many instances of Lua interpreter as you want. --[mfi].

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Lua is quite fast: http://www.bagley.org/~doug/shootout/ but doesn't seem to have all the nifty features that other languages do.

On another hand, Lua '''does''' support full [co-routines]; Roberto is a 
sometimes-cited theoretician of [concurrency].
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[CL] sometimes collects Lua references
[http://www.phaseit.net/claird/comp.lang.misc/lua.html].


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While Lua began with an emphasis on industrial automation and allied areas, it's now so significant in game development
that ''Game Scripting Mastery''
[http://notyet]
and ''Game Programming with [Python], Lua, and [Ruby]''
[http://notyet]
provide substantial introductions to it.

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An article about Lua's byte code engine: http://www.osnews.com/comment.php?news_id=9817

Apparently they moved from a stack based to a register based engine and saw big performance improvements.

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[Category Language]