Yes, I am, because I believe that men and women should be treated equally under the law.
I'm always sure to distinguish between feminists (whom I tend to agree with) and feminazis (who are crazy).
I can't say that I've done much to advocate, since men and women are already pretty much equal under the law. I have joined several support groups for women, though, despite being male.
I have, though, done much in support of men's/father's rights, as men are currently screwed over in the legal system compared to women in many instances, particularly in custody cases and with alimony. The reason I am more vocal about this issue, aside from being male and afraid of being screwed over, is because many people (feminists included) don't recognize this problem or, worse, are okay with this blatant sexism.