MeFi taught me html!
It was September 2000. I had been a member for a month and didn't understand how people made their text link to pages, and do italics, and the like. So I asked on Metatalk. As a result, Metafilter is the first place i had even heard of html. And from there it took about two months to get a blog and learn some stuff that, while not even close to professional, certainly makes me look like the tech-savvy member of the family.
As a bonus, that thread was my introduction to some people that would become my best online friends. I have met up with MeFi people in Chicago, St. Louis, San Francisco, and basically anywhere I go now I have friends. Thanks, Matt!