People who don't like what you have to say, but can't find anything wrong with it, so they feel to resort to personal insults (a lot of them on this board - don't feel you have to make a bad joke about this by responding to this with personal insults).
Religious lunatics (including many who don't think of themselves that way- homophobes, godwetrusties, etc)
SCO, microsoft, DRM, corrupt corporate legislation (DMCA, INDUCE, etc).