You should try LuaJIT, it is a drop-in replacement for Lua 5.1 and it has a lot of great features and exceptionally good speed for a dynamic language implementation thanks to the JIT compiler.
Originally Posted by Ardje
LuaJIT's FFI is amazing, the calls to C get inlined by the JIT compiler making C calls very fast.
LuaJIT also has a bit library that gets optimized by the JIT as well. Calls to bit functions will get optimized out by the JIT compiler and make bitwise operations very fast.
local ffi = require("ffi")
local n = ffi.new("int64_t") -- create 64 bit integer
IMO Lua with LuaJIT is one of the best, if not best, dynamic language implementations currently available.