Scheme

From GPWiki

Scheme is a language derived from Lisp. It is designed to be small and elegant, which it is. On the other hand it is not very suited for practical programs in its standard form, because it hardly contains any methods for doing I/O. Most interpreters (quite a few exist) fix this by providing extra features, and some effort to standardize these is in progress ([1]). If you've never programmed any Lisp-like language before, I recommend learning one just to get a feel for some cool programming techniques it allows (and Scheme is a good candidate, it is far simpler than any other lisps).

Some online books on Scheme:

Teach yourself Scheme in fixnum days 

A basic introduction to the language.

The Scheme programming language 

A complete and thorough description of the language.

The revised⁵ report on the algorithmic language Scheme 

The languag standard, much more readable than most language standard documents, but somewhat cryptic at points.

Structure and interpretation of computer programs 

A rather technical, but excellent, book about scheme and programming in general.

Good implementations:

MZScheme and DrScheme 

A rock-solid, feature-rich implementation, with a graphical IDE and (incomplete) built in OpenGL support.

Scheme 48 

A more minimal, but still rock-solid implementation.

Chicken 

A Scheme-to-C compiler.