Here is a link of existing ontologies created by other people: http://www.daml.org/ontologies/ ... it's helpful to see how other people visualise various problems.
I was having really hard time understanding what an ontology really is. I read lots of article to get a feel of it and here is a list of good articles that I will recommend to everyone who still has no clue about the topic.
http://www-ksl.stanford.edu/kst/what-is-an-ontology.html (Ontologies for the beginners, really explain what it is.)
http://www.semanticweb.org/knowmarkup.html (Putting everything XML, Ontologies and Semantic web together.)
5 students at the University of Amesterdam wrote an article that compared 5 different tools to assist users in the creation of Ontologies. These tools were evaluated based on the User Interface, Ontology-related features, and how easy it is for multiple people to collaborate on the ontology at the same time. They found that no tool is perfect (is it ever?), and recommend different programs based on the previous knowlege that the end user has with creating Ontologies.
Even though the article is a couple years old, it still gives you a couple of programs to start out with if you are looking for tools to create Ontologies, and it gives you an idea of items and features that you will probably want to evaluate if you find other tools that can be used as well.
http://www.semanticweb.org/knowmarkup.html
This site has a very interesting paper on markup languages and ontologies. It gives a very clear picture on XML,Sematic web and ontologies. Ontologies are explained with examples and in much detail in this paper.It is a very interesting site to understand the relationship between ontologies and semantic web.
The following is a link to a project with the objective of developing "tools and methods for supporting knowledge management relying on sharable and reusable knowledge ontologies". It is managed by a consoritum of members with strong interest in a variety of aspects of Knowledge Engineering.
Bob