ChaiScript

I use ChaiScript in the Open-Transactions project, and so far it has accommodated all of our needs.

ChaiScript integrates very easily with C++, and the language itself is cool, and has lots of features. (Classes, lambdas, variants, etc.) It comes with a "standard library" including wrappers for common C++ classes. It's pretty powerful in terms of passing various data types back and forth, handling exceptions, wrapping classes and overloaded functions, etc.

The documentation can be maddeningly sparse, but the project maintainers pick up the slack on their forum, where they have been responsive, attentive, and have even added language features upon request. (They added "switch" upon request, for example.)

Having corresponded with the maintainers from time to time, I feel good also about their attention to optimization and static/dynamic analysis (Valgrind, etc.)

Bottom line? Excellent scripting language, and a quality implementation!