Let's see, off the top of my head, these are the groups you generally run into throughout SL. Some are large, some are small, all have an impact. They're subcultures, really, not just groups.
(opinions of cynical oldbie follows, possibly tongue in cheek)
1)
BDSM folks. Whips and chains, with the slaving, and the biting, and the mhay-mhay. Sub-section of this group called Goreans (or followers of Gor), who seem to glorify women as property and fight with large swords.
Location: Goreans have a sizable continent, with the main sim being (I think) Port Kar. Very roleplayeresque.
Countermeasures: universal sufferage, gunpowder
2)
Furry folks. Two general types: PG and not-so-PG. Most are very nice people, and very smart.
Location: Their main hangout is Luskwood, or I think the Forest (is that still around?)
Countermeasures: SomethingAwful.com
3)
Misc. roleplayers: Ageplay, kidplay, ponyplay, playplay, kid'n'play... whatever. There are a lot of roleplayers around SL, and they each have their quirks.
Locations of hangout: Skyboxes 500m up in the air.
Countermeasures: staying below 500m
4)
Jedi: Mostly 12 year olds who haven't been shipped off to the teen grid yet.
Hangouts: Sandboxes.
Countermeasures: age verification, bedtime, school hours
5)
Gunplayers: These guys script guns, weapons, turrets, and so on, and have wars and all that. Mostly harmless, regardless of the size of their chaingun; downright impotent if you sit on a box.
Hangouts: Sandboxes, Jessie.
Countermeasures: Lindens, abuse reports, plywood cubes, safe areas
6)
Mafia, Mafioso: Likes to roleplay as "The Mafia". Usually Americans speaking broken, improper Italian. They enjoy "warring" with each other, cursing every other word, and "staking out" their "turf". Usually blinged out. A lot of them are rumored to be refugees from The Sims Online (unsubstantiated) Totes guns around.
Hangouts: There's a few private sims they lurk about in (Little Italy?). If you see any vaguely Italian group names, chances are it's a mafia.
Countermeasures: Really being from Italy, big words, turning off particle effects
7)
W-hats: Folks whose only common denominator is they read SomethingAwful.com. Some are nice, but some are really, really big assholes. Major bent against anything not considered mainstream. Very testy. Amused by dead baby jokes.
Locations: Baku
Countermeasures: none so far, although letting them crash the grid a few times stops them for a few hours while the grid comes back up. Even Lindens seem generally defenseless against them.
Land Barons: These folks buy and sell land. People villify them much like Generation Xers villify "The Man". Lucrative career, also fairly risky. One of them makes (allegedly) six figures in $US a year.
Location: Almost any piece of land for sale
Countermeasures: $L inflation, grid instability, communism
9)
Forum folks: Imagine a ball of sarcasm, dripping with words soaked in acid. These people are in the core of this ball, producing enough venom to drop a moose. Small subsection of the population, but an even distribution from all walks of life. Perhaps 5% of the population participates in the forums.
Location: Forums.Secondlife.com
Countermeasures: Jeska Linden, flame wars, General Forum
10)
IRC Folks: Snarky. Half of them just idle. Secretly pulls the puppet strings of SL society. Stays crunchy in milk.
Location: #Secondlife on Efnet
Countermeasures: Netsplits, tiramisu, any link by Belaya.
I think that's all I can think of.
