If you need to add a scripting language to your application, Neko provides one of the best tradeoffs available between simplicity, extensibility and speed.
No, dude. Lua does. It's simpler, faster and smaller than Neko. It's also more portable than Neko (using spare bits as opposed to tagging on values == a lot less portable, yo). Unlike Neko it has lexical scoping and the associated closures, plus it has coroutines, which are awesome.
The big problem with Lua is it takes being small and portable so seriously that it doesn't have any support for Unicode either. Ghey.