HN. I joined the day after it opened for public use (it had been in beta for YC founders for about 6 months previous).
FictionAlley.org. I started lurking when they had a couple hundred members, and joined as user 1881. Joined the staff a couple months later, when we were at around 2500 registered users, and when I left the staff 3 years later, we had over 100,000.
Dropbox. Like you, I remember being super impressed by Drew's little demo on the HN forums. I actually had lunch with Drew, Arash, and Aston right after they got their Sequoia funding to discuss joining the company as employee #2. Had my own startup at the time and wouldn't leave my cofounder; I keep wondering if this'll be one of those decisions I'll regret forever, but OTOH the failure of my startup afterwards led me to Google, and that's been a pretty good experience too.
FaceBook. In September of 2004, a couple of my friends were all excited about this new social network that had just expanded to Amherst from the Ivy League. The founders had apparently just moved out to Silicon Valley, gotten some angel funding, and were shopping the company around to acquirers and VCs. I joined, found it kinda "meh", but friended all my friends (and a few people I met just once or twice at parties). At the time, they had profile photos, contact info, and the Wall, but nothing else. No private messaging, no chat, no photo-sharing, no apps, no events, no groups, etc.
Avici (remember them? They sold the massive routers that Enron Broadband and many of the major ISPs of the first dot-com boom used). Sometime in the summer of 1995 or 1996, we were sitting by the pool with the family of one of my friends from middle school, and talking computers. My friend's father said he and a bunch of his coworkers from BBN were founding a new company. They'd wanted to do these multi-terabit/second routers for years, and given how the Internet was exploding, it was now or never. They IPO'd at a market value of over a billion dollars in 2000, my friend's dad cashed out as soon as the lockup was over, and then the company promptly tanked.
Akamai. I was in a support group for gifted & talented kids with Reid Barton, the 4-time IMO gold medalist, when I was a teenager. Sometime around 1998, my dad was talking with his mom about what he was up to. He'd been working on the Cilk programming language at MIT, and one of the professors in the department was leaving to found this Akamai startup. Reid was going to join him as an intern. The irony is that I waited until the dot-com bust, after Danny Lewin was killed on 9/11, then picked up a whole bunch of Akamai shares on the open market at a price lower than the strike prices of virtually all the employees. That pretty much funded my first startup.