I Was never really a huge part of any communities, but one of the first servers I played on really often was TrU 24/7 2Fort. I played nothing but Sniper hoping to get better aim by practicing with it so much. That didn't really work out very well.
Then there was clan AI. They only had one server and a small community, but it was fun. I think a large part of the community were Mormons, so everyone was super nice and polite. Unfortunately it got harder and harder to populate the TF2 server, and it eventually died. I think the clan is still around, but it's moved on to other games.
There was also Nom Nom Nom, and the Parlor Room Jesters.
Probably the biggest though had to be Ubercharged.net. I spent a lot of time on what was known as the UC1 server. Unfortunately shortly after I started playing there, the owner decided to give it up and tried to find someone else in the community to transfer ownership to. This also happened to coincide with the peak of the MLP popularity thing. That split the community pretty badly, and when the transfer of ownership was done, everyone had pretty much moved on.