I can't recall the true first time I used OpenCV because it's simply so embedded in all image processing work. I've backed the project, and my company has contributed as well.
I just scrolled back in my email - 9/7/2000 was when I subscribed to their email list, I think that's when I first heard about. I have a vague memory that before OpenCV there was an Intel thing we sometimes used but it had some weird usage restrictions.