The "extremists" of the animal rights movement would be the Animal Liberation Front (ALF) and the Animal Rights Militia (ARM). Both of them fight for animal rights through direct action (physically freeing animals who are currently imprisoned, defacing property, etc.), but the ARM goes farther and has no problem hurting anyone in their way to get what they want, too!
First, though, let's talk about the ALF.

The Animal Liberation Front
Here is part of the credo of the ALF:
"The Animal Liberation Front (ALF) carries out direct action against animal abuse in the form of rescuing animals and causing financial loss to animal exploiters, usually through the damage and destruction of property.
The ALF's short-term aim is to save as many animals as possible and directly disrupt the practice of animal abuse. Their long term aim is to end all animal suffering by forcing animal abuse companies out of business.
It is a nonviolent campaign, activists taking all precautions not to harm any animal (human or otherwise)."
Activists independently take action for their cause and then anonymously claim that their action is a part of the ALF. There is no official "membership" card or fee to be part of the ALF, nor is there a leader; it's more a collective mass of actions that add up together to form the group. The only requirement is that the action follows the guidelines of the ALF and that the people committing the action are either vegan or vegetarian. Their website can be found here: http://www.animalliberationfront.com/index.html.
Photo Courtesy of www.animalliberationfront.com
My favourite ALF-er is Rodney Coronado. He has done some questionable (I won't say "wrong", because it's a difficult subject) actions, sure, but after reading the book Operation Bite Back about some of his actions (amazing book-- you should read it), I can't help but think he's an amazing person.

The Animal Rights Militia
The ARM is probably a part of the ALF or at least related to it, created by those animal rights activists who are sick of behaving non-violently towards their opponents. The ARM has threatened animal abusers with death and claimed that they have poisoned certain foods from companies that they disagree with (only later to say that they never actually poisoned anything-- they just pretended to so that the company would be forced to recall the product, causing financial damage).
In terms of governance, the ARM operates in the same manner as the ALF-- no leaders, only individuals taking action and claiming that their action was a part of the ARM.
There isn't much information out there on the Animal Rights Militia. On Wikipedia, it is stated that it is dubious that the ARM even exists, and perhaps it was created by the Animal Liberation Front to distract the authorities from itself. Keep in mind, however, that I can see no other evidence pointing to this after extensive research on the web. Let it suffice to say that the ARM is mysterious indeed...

      Why the Sea Shepherd Conservation Society Doesn't Count on This List
The Sea Shepherd Conservation Society is a fabulous environmental organization dedicated to saving whales from illegal whalers. I think they're great, but they aren't an animal rights organization; they're an environmental organization. If you want to learn more about SSCS, please visit their website at www.seashepherd.org. I highly recommend that you do. :)

Extremists: Good or Bad?
Just because I don't want to join the ALF doesn't mean that I think they're all a bunch of lunatics. In fact, the ALF probably has done a lot of good in exposing the horrors of the animal industries. I only say that I wouldn't join them personally because 1) I have some reservations about the philosophy behind it (see below), and 2) I have other plans for my life.
As for the ARM, I don't support its position at all. I suppose it depends on whether you believe it's okay to threaten and potentially injure people so that you can save animals-- and I know for sure that my instinctive response is "no, you shouldn't kill anyone to save anyone else, unless of course you're standing right there watching them kill your best friend or your best-friend-dog or something".
But back to the ALF-- the non-violent version, that is. It's important to note that most ALF-ers do appear to be very against violence, which is a good thing. I LOVE making lists, in case you haven't noticed by now, so here's a handy-dandy list of the pros and cons of the ALF.

Pros of the Animal Liberation Front
  1. They don't fool around with naked demonstrations and things, nor do they target minority groups in their campaigns, unlike certain other animal rights organizations *cough cough*. (Unless you count vivisectors as a minority group, that is!)
  2. They want real results NOW-- not just whining and giving "you-can-do-it" go-veg speeches and the like. (I'm not critisizing public-awareness forms of activism. I participate in public-awareness activism myself, after all-- through my blog, 'zine, and other projects.)
  3. They financially damage industries that exploit animals.
  4. They get people to think about their roles in causing animal exploitation (the people exploiting the animals, and the people supporting it with their daily choices), sometimes causing people to alter their lifestyles to be more compassionate.
  5. They remind us that life is real, here and now-- there is danger and urgency, and a real world beyond legislation and normality. We mustn't be complacent just because we have it easy in the developed world.
Cons of the Animal Liberation Front
  1. When they free animals, often the animals will just be replaced with new animals bought from breeders, thus causing more animal suffering.
  2. The activists who are part of the ALF are sometimes captured and arrested. Rodney Coronado was arrested multiple times, and even during time when he wasn't in prison, he spent a lot of time on "probation period" in which (it appears that) he was not allowed to do environmentalism or animal rights activism. That would be so incredibly heartbreaking if it happened to me, not to mention the fact that it's bad for the movement because we've got one less pair of helping hands.
  3. Many people see the people in the ALF as dangerous extremists, and may come to see animal rights and veganism in a bad light if they associate it with the ALF.
  4. Although they claim to be non-violent, their actions are borderline violent-- arson, spraypainting, wrecking things; this does not contribute to a safe, democratic society. (Of course, many of the ALF-ers are anarchists, so they don't care about this last point much).
Okay, that's all for this week. Next week, I think I'll take a stab at explaining why silk isn't vegan, and what alternatives there are out there instead.