Scheme is a dialect of LISP which attempts to keep the semantics clean and simple with few divergent ways to express something (which is a diametrically opposite tack to Perl, which prides itself in having "more than one way to do anything").
Scheme also has its own variants, such as GNU Guile, Racket, and Pyramid.
- MIT Scheme page
- Current version of the Scheme standard, R6RS
- Previous version of the Scheme standard: R5RS
- Working groups and drafts for the next version of the Scheme standard, R7RS, here
- List of Scheme resources, papers and implementations 
- Structure and Interpretation of Computer Programs (aka. "The Scheme Bible") available here (based on R5RS)
- The Scheme Programming Language, Fourth Edition, by R. Kent Dybvig (free online book)