The nice thing about Lua is that it can easily be combined with compiled languages. If you identify the hot path in your program, you can implement them in a more efficient language.

The C interface to ruby is just superb. It provides a very simple and first class access to the entire runtime and all it's features. It's absolutely my favorite language to write extensions in.

Lua is probably my second favorite. It's marred by it's initial creation as a stack based interpreter and requires more calls and contortions to achieve the same effects as you would in any other language; however, once you understand a handful of useful primitives it's quite easy to intuit the correct set of calls for any use case.

The blend of dynamic language with underlying compiled extensions is vastly underappreciated. I suspect it has a lot to do with the difficulty of packaging and distributing these extensions into current virtualized and cloud environments. Which is a pity given the extreme combination of flexibility and efficiency that it otherwise unlocks.