XParam
Parameter handling library. Extendable, type-safe, non-intrusive, object-oriented tool for general-purpose object serialization in C++. Good for parsing command-line parameters, cross-program and cross-platform communication, object streams, and as a plug-in framework (using polymorphism).
http://xparam.sourceforge.net/

Digitool, Inc.: Object Logo
Object-oriented, extends abilities and performance of traditional Logo, useful for users of all ages, elementary school students to professional programmers; potent education tool and means to explore programming, general and more so object-oriented. No longer supported.
http://www.digitool.com/ol-specs.html

OO Example Code
Example using object-oriented polymorphism in object-oriented and non object-oriented programming languages.
http://onestepback.org/articles/poly/

Guile Object Oriented Programming System: GOOPS
Object-oriented extension to Guile; very close in spirit to Common Lisp Object System, CLOS (CLtL2), but adapted for Scheme language; gives full OO system with multiple inheritance, generic functions with multi-method dispatch.
http://www.gnu.org/software/goops/goops.html

Phoenix Object Basic
A unique object-oriented RAD tool for Linux.
http://www.janus-software.com/

What is Object-Oriented Software: An Introduction
Simple explanations for object newbies.
http://www.softwaredesign.com/objects.html

Blaise
An object-oriented scripting language with Object Pascal-like syntax. [Open source, MPL]
http://blaise.sourceforge.net/

Umapathy, N.S.
Interested in thinking objects, and object computing and methodologies, some useful links. Think Object-Oriented.
http://www.geocities.com/nsupathy/

OTcl: MIT Object Tcl
Compact extension to Tcl/Tk for object-oriented programming, from MIT.
ftp://ftp.tns.lcs.mit.edu/pub/otcl/README.html

Law of Demeter
Information on this object oriented style rule which aims to limit the number of objects each object interacts with.
http://www.ccs.neu.edu/home/lieber/LoD.html