Another example is this syllogism, which alludes to Alan Turing's homosexuality:

Turing thinks machines think.

Turing lies with men.

Therefore, machines don't think.

(Note the equivocation in the use of the word "lies".)

A common form is an attack on sincerity. For example, "How can you argue for vegetarianism when you wear leather shoes ?" The two wrongs make a right fallacy is related.

A variation (related to Argument By Generalization) is to attack a whole class of people. For example, "Evolutionary biology is a sinister tool of the materialistic, atheistic religion of Secular Humanism." Similarly, one notorious net.kook waved away a whole category of evidence by announcing "All the scientists were drunk."

Another variation is attack by innuendo: "Why don't scientists tell us what they really know; are they afraid of public panic ?"

There may be a pretense that the attack isn't happening: "In order to maintain a civil debate, I will not mention my opponent's drinking problem." Or "I don't care if other people say you're [opinionated/boring/overbearing]."

Attacks don't have to be strong or direct. You can merely show disrespect, or cut down his stature by saying that he seems to be sweating a lot, or that he has forgotten what he said last week. Some examples: "I used to think that way when I was your age." "You're new here, aren't you ?" "You weren't breast fed as a child, were you ?" "What drives you to make such a statement ?" "If you'd just listen.." "You seem very emotional." (This last works well if you have been hogging the microphone, so that they have had to yell to be heard.)

Sometimes the attack is on the other person's intelligence. For example, "If you weren't so stupid you would have no problem seeing my point of view." Or, "Even you should understand my next point."

Oddly, the stupidity attack is sometimes reversed. For example, dismissing a comment with "Well, you're just smarter than the rest of us." (In Britain, that might be put as "too clever by half".) This is Dismissal By Differentness. It is related to Not Invented Here and Changing The Subject.

Ad Hominem is not fallacious if the attack goes to the credibility of the argument. For instance, the argument may depend on its presenter's claim that he's an expert. (That is, the Ad Hominem is undermining an Argument From Authority.) Trial judges allow this category of attacks.