Every once in a while I'll discover a little corner of the internet that has some tight-knit community that I'm not a part of. These little communities can spring up from a variety of common interests, and they sometimes (but not always) consist of groups of people that form communities in real life: discussion forums for people interested in running, the television show Lost, sushi restaurants in Chicago, or all manner of other things; community center or church websites with discussion boards; baking-themed blogs with copious interlinking; the people who show up on Yahoo! games to play Literati between 1am and 3am every night. Pretty much if you're interested in it, someone else on the internet is interested in it too. And there's a whole wide array of possibilities for when and how you choose to connect, and you can start, join, or otherwise affiliate with as many as you have time for or interest in. What started with chatrooms and Usenet has really hit the bigtime.
For most of us, the little communities that we find on the internet are the ones of which we're naturally already members. I like restaurants and cooking, so I get to know people on egullet. I like running, so I read shoe review boards and occasionally post. I'm on linguistics distribution lists. You probably have your own versions -- and since you're reading this, maybe you and I even belong to some of the same online communities (especially if your blog is linked on the left!).
But every once in a while, I'll encounter some very close-tied community on the internet that I'm not a part of. You're all familiar with the lurker phenomenon, where you read distribution lists or discussion forums without really posting yourself. But most of the time the conversations you're eavesdropping on are conversations among people who at least to some extent would recognize you as someone with shared or overlapping interests. It's far rarer and more interesting, at least for me, to bump into some flourishing online community that's chatting up a storm where those people don't have any real-life connection to anything you're involved with. The people you observe become more like characters in a novel, or in a movie, or even on some trashy night-time drama on FOX. Who knows what will happen next? And you get to watch it all from the privacy of your own monitor.