I found MetaFilter through the tiny blog community that existed at the time.
Powazek, Prolific, Megnut, Kottke, Zeldman, Jack Saturn and Rebecca Blood all led the way to the site, and I joined up.
9/11
It's not a fond memory exactly, but 9/11 is when MeFi clicked for me. I was in San Francisco on a holiday, a lone European in California. When 9/11 happened I was lost, unsure of what to do, or even what had happened. Metafilter was vital to me in understanding what had happened, and more importantly, how it had impacted on the country in which I was visiting.
More than any other site, MetaFilter showed me how powerful online communities can be.