DrScheme is an interactive, integrated, graphical programming
environment for the Scheme, MzScheme, and MrEd programming languages.
DrScheme incorporates several features including highlighting of
the source of syntax and run-time errors, support for multiple
language levels, an algebraic stepper, objects, modules, a GUI
library, TCP/IP, and much more. It includes an extensive, hyper-linked
help system called Help Desk, available from the Help menu.
DrScheme Jr is a textual version of DrScheme that offers some of the
benefits of DrScheme, but without development tools or graphics. 

WWW: http://www.cs.rice.edu/CS/PLT/packages/drscheme/