Buffalo Ontology Site: Information on ontology, on the history of ontology, and on contemporary ontology and its applications.
Metaphysics of Quality: Robert M. Pirsig deals with the fundamentals of existence and attempts to provide a more coherent system for understanding reality than current paradigms allow.
Ontology. A Resource Guide for Philosophers: Resources on the development of ontology, especially in the twentieth century.
PhilSci Archive: Idealist Philosophy: What is Real ?: Paper by Axel Randrup defending idealism and arguing that materialism leads to logical contradictions, which closes by considering the implications of this for religion.
Qualia: The Knowledge Argument: Aims to establish that conscious experience involves non-physical properties. It is one of the most discussed arguments against physicalism; from the Stanford Encyclopedia by Martine Nida-Rümelin.
Stanford Encyclopedia of Philosophy: Abstract Objects: Survey of attempts to draw the distinction between concrete and abstract objects; by Gideon Rosen.
Stanford Encyclopedia of Philosophy: Events: Survey of philosophical views on the character and status of events; by Roberto Casati and Achille Varzi.
Stanford Encyclopedia of Philosophy: Existence: Survey article by Barry Miller.
Stanford Encyclopedia of Philosophy: Holes: An interesting case-study for ontologists and epistemologists.
Stanford Encyclopedia of Philosophy: Physicalism: Discussion of the thesis that everything is physical; by Daniel Stoljar.
Stanford Encyclopedia of Philosophy: The Identity of Indiscernibles: A principle of analytic ontology first explicitly formulated by Leibniz. It states that no two distinct substances exactly resemble each other.
Stanford Encyclopedia of Philosophy: Tropes: An article describing tropes; by John Bacon.
The Metaphysics of Blue: On the nature of consciousness - looking for soul in a soulless world.
The Shift of Aspect in Language and Thought: Notes on the relational element in the assertion that things exist. Patterned on Wittgenstein's Philosophical Investigations.
What is Ontology?: Collected definitions, from leading philosophical reference works and from philosophers from Wolff to Husserl.
Why Does Anything Exist?: Sequence of short essays on this topic.