Enhancing Object-Based System Composition through Per-Object Mixins
The paper introduces per-object mixins as a new object-level language construct for object-based system composition. Per-object mixins are implemented in the scripting language XOTCL as a straightforward extension of its method chaining mechanism and enables the developer to mix classes into the precedence order of an arbitrary object.
http://wwwi.wu-wien.ac.at/~uzdun/publications/xotcl-mixin.pdf

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

Object-Z Page
Object-oriented extension of Z formal specification language, developed by research team at University of Queensland. Brief description, links, FAQ, tool support, publications, book reference.
http://www.itee.uq.edu.au/~smith/objectz.html

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

Logtalk
Logtalk is a free object-oriented extension to the Prolog programming language. Integrating logic programming with object-oriented and event-driven programming, is compatible in most Prolog compilers.
http://www.logtalk.org/

Dynace
Blake McBride - Arahant.Com - Nashville, TN - Dynace - VMEM - LISPF4 - Spinoza Society
http://algorithms.us/

Oclarity
Enhance your Rational Rose models with OCL expressions.
http://www.empowertec.de/products/rational-rose-ocl.htm

Comega: Cω
C# extension, in two areas: Control flow extension for asynchronous wide area concurrency (was Polyphonic C#); Data type extension for XML and table manipulation (was Xen and X#). [Microsoft Research]
http://research.microsoft.com/comega/

FoxDataObjects
An object-relational mapping extension library that enables developers to create fully scalable business classes that are persistent to relational databases.
http://www.foxdataobjects.com/

Object Primer
The Application Developer's Guide to Object-Orientation. An introduction to object-oriented analysis and design techniques. By Scott W. Ambler, SIGS Books.
http://www.ambysoft.com/theObjectPrimer.html