It's a great programming language. Scheme is minimal, yet extensive in what it offers, it has a minimal syntax, the language spec is an easily-understandable, quite short document, and once you know the language, it is a nice and educational next step to implement it yourself. There's a reason why SICP uses Scheme.

Also, Chicken Scheme specifically because it is easy, small, portable, has a good extension mechanism, and an active and awesome little community.