Object Oriented Programming: A guide for the beginner, from Modula-2 to Java
Tutorial teaching basics of object oriented programming. Tailored to no one specific language, but examples are in C++, Java, Modula-2.
Growing article, with links to many related topics. [Wikipedia]
Objects on the Web: Designing, Building, and Deploying Object-Oriented Applications for the Web
By Ron Ben-Naton.
Planet Source Code
Lets OOP programmers submit code for review by other programmers; many source code samples to help educate beginners on many concepts; contests where programmers vote for the most efficient, useful code recently submitted.
WebReference.com: Class-Based vs. Prototype-Based Languages
Brief explanation by Yehuda Shiran, Ph.D.
What is Object-Oriented Programming?
Definition, lists some links for more information. [Webopedia]