For the record, By Any Means Necessary, the group whose appearance on FBI watch lists motivated Diane's post post, isn't the most-cuddly bunch out there. BAMN is a front organization for the Trotskyite Revolutionary Workers League. And calling themselves a peace organization is a recent framing device; previously, they've made their hay on "defending" affirmative action.
Hard to believe that something deserving to be called a Communist front-group exists in the U.S. these days, but it's true. In a past life, BAMN activists worked to infiltrate a pre-September 11, 2001 leftish-student conference that I helped organize, once it became clear that they weren't going to make much headway at the Rainbow/PUSH conference held across town. They didn't get far with us either, beyond briefly seizing the mic at a plenary session. But it gave a bit of a scare for the few of us in the know.
Nathan Newman wrote a good piece on the group a couple of years ago. There's more info elsewhere. Of course, those who really believe in civil liberties must hold fast even when the most repugnant groups exercise them. BAMN is really no terrorist organizationand certainly isn't one in the usually-understood context. But they probably aren't the most strategic group to pick as a civil liberties poster child.