Typed Scheme is a typed dialect of PLT Scheme. It integrates with modules written in other PLT dialects, and provides a type system designed to support common Scheme idioms. Typed Scheme is available as part of PLT Scheme version 4.
The manual provides an introduction and reference for Typed Scheme.
More technical material on Typed Scheme, including an Isabelle/HOL formalization of the core of the type system, is available from the various papers.