Danny points to Ontology Portal - The Suggested Upper Merged Ontology which has been updated. SUMO, CYC, WordNet and similar efforts are always kind of fun to explore but I'm always left with a feeling of disappointment.
For example, the SUMO page above has a search box with a deliberately provocative default search term: Buffalo. It waits there quietly whispering "come on, type something if you dare; I'm more than just prehensions and qualities and actualities, I know about buffalos and real things like that; come on and try me." My eyes swept the desk looking for something obscure: CD, screen wipes, coffee mat, translucent flexible gap filler, Sony-Ericsson K500 docking cradle. In the end I stopped being vindictive and tried something pretty commonplace: Paper. It gave me some definitions from WordNet and some links from each definition to SUMO concepts. Clicking through to Artifact reveals some very interesting work including some inferences such as "artifacts are the result of making" and "plans express artifacts". Looks pretty good. Then I noticed at the top of the page the definition of artifact: "A CorpuscularObject that is the product of a Making.". Corpuscular! The only other time I've seen that word has been when reading about Isaac Newton. Clicking on CorpuscularObject was when the disappointment started to set in. Each page has a link to a graph of the sub/super class relationships for each concept. The graph for CorpuscularObject looks like this:
Can you spot the deliberate mistake? Why is nest expressed at this level? I did wonder that perhaps it was some meaning of the word that I wasn't familiar with but no, a nest is "Any structure which is created by nonhuman Animals for the purpose of giving birth to their offspring." Browsing SUMO turns up lots of these kind of inconsistencies.
Yes, I understand it's not finished and it's only "suggested", however it strikes me that there's a certain arrogance in what these grand, all encompassing ontologies set out to achieve. They seek to lock down and define all essential aspects of the world in a central and dictatorial manner. Yet the world doesn't work like that. Meanings change from time to time, place to place, mind to mind. When I think of a nest, I rarely imagine sticks and twigs, instead I picture my son surrounded by beanbags, nestled down, reading a book. My humble advice is to keep it small. Start with something manageable and grow it just enough. Let different parts of it evolve at different rates just like language does. Let small pockets of expertise grow their own ontologies, then figure out how to work with them. Identify the common terms and use those as stepping stones between domains. Decentralise and diversify. The result will be a richer, relevant and more useful web of knowledge about the world.