Other small, embeddable schemes that are worthy of notice:

* S9fes: https://github.com/bakul/s9fes

* Chibi: https://github.com/ashinn/chibi-scheme

Both are just a couple of files to add to your project, and easily extended with good built-in features.

I've also had a lot of luck embedding Chicken. It's quite straightforward to do, and with the boon of being able to compile Scheme to C to include directly.