The following tree is extracted from Alex Shinn's post to comp.lang.scheme.

  • CLOS: Common Lisp Object System, based on CommonLoops and New Flavors, winning out in the CL standardization over Object Lisp and Common Objects. I know nothing of those 4 original systems.
    • TinyCLOS: Subset of CLOS written in pure Scheme, including a meta-object protocol (MOP). Classes, generics & methods all 1st class. Native OO system for Chicken.
      • SOS: MIT Scheme's native OO system, mostly differs in naming and interface. No MOP via meta-classes but supports computed methods.
      • RScheme: Based on CLOS and Dylan. The object systems in Gauche and RScheme are remarkably similar.
      • STklos: Native TinyCLOS implementation. Class redefinition. Don't need to predefine generics (complicates module interaction).
        • Gauche: Well integrated with native libraries. Collections API. MOP utilities.
        • GOOPS: Guile's native OO system, also well integrated. Very well documented.
    • SISC: Similar to TinyCLOS (possibly a conscious influence though I can't find any lineage in the docs). Limited multiple inheritance.
    • Meroon: Another CLOS derivative, similar to TinyCLOS but more featureful (also has stripped down version Meroonet). MOP. DSSSL support. Excellent documentation.
      • Bigloo: Native implementation based on Meroon, has 1st class classes, generics & methods. No MOP. Fields can be typed with native C types for efficiency.
    • Swindle: PLT Scheme's. (see MzScheme).
  • YASOS: Yet Another Scheme Object System, a portable OO system distributed with SLIB. Works in terms of dispatching methods (called operations) on arbitrary Scheme values (which can inherit from arbitrary Scheme values - there are no classes). Includes a collections API.
    • MOS: Macroless Object System, removes macros (and thus uses different API) from YASOS but keeps the idea of methods on objects with no classes. Distributed with SLIB.
  • MzScheme: System of classes, objects and methods, all 1st class Scheme values, but no MOP. Single inheritance with interfaces and a public/private distinction similar to the C++ & Java style of OO. Uses explicit send syntax to call methods, although a with-method syntax is provided to look like CLOS methods. Excellent interactive documentation.
  • Kawa: Access to Java OO system. Methods defined statically with classes. Provides "this" syntax.
  • SICP: Chapter 3 of The Structure & Interpretation of Computer Programs describes a very simple dispatch closure based OO system. Some of the smaller OO system's in the Scheme repositories follow this style, most Schemers write such an OO system at some point, and many Schemers use this style without any sugar when they want portable code (see much of SLIB).