GLOS is a library of Scheme functions and macros that adds two primary bits of functionality to Scheme:
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Types: top, bottom, (and? t1 t2 ...), (or? t1 t2 ...), (== val), predicates (boolean-returning functions), and record subtypes.
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Dynamic Dispatch: When a generic function is called, it selects the most applicable method based on matching the types of the arguments against the specializers of each method. This is multiple dispatch, as opposed to the single dispatch of C++, Java, Smalltalk, etc. GLOS also supports method combination (before, after, and around) methods.
Currently the code is specific to Scheme48 (see www.s48.org), but only very slightly so. Ports welcome.
To load GLOS, fire up scheme48, open the file test.scm, and paste the first five lines from test.scm at the scheme48 prompt.
Some description of GLOS is in the paper ref-dyn-patterns.
Also, slides in proglangsandsofteng work through some examples using GLOS.