Barry Smith, a philosopher turned ontologist, has a two-day course that he's made available online that introduces the concept of ontology and delves into their history, practical application, and connection to computer science. For those wishing to get a firm grasp on what ontologies are and how they're used, I highly recommend that you take the time to give these lectures an in-depth listen.
I find Barry to be a captivating lecturer capable of distilling some very abstract concepts down to understandable, actionable pieces of knowledge. I saw him speak to a group of developers as a part of the National Center for Biomedical Ontology's recent meeting, where he urged all developers to at least give his course some attention. From other people in the ontology community I understand that he has a very particular viewpoint on the ontology world. But I found it useful and hope others will too. Any pointers to other educational pieces will be welcomed in the comments.
The videos, available for streaming below, are part of a two-day course that Barry teaches. Barry indicates that they are free to use in any capacity, so I took the liberty of uploading them to Viddler (the only free streaming video site I could find that would allow long videos with no special account).