Just saw this one used on facebook. Guilt by Association is type of ad hominem which attacks a person's argument by comparing it's similarity to the views of other (generally disfavored) people or groups. The intention of the person using this fallacy is to have the argument rejected due to this association instead of facing the actual argument.
The basic format can be seen as; Person A argues for position 1. Unsavory person (or group)B also argues for position 1. Therefore position 1 must be wrong. Here is a silly example which makes the fallacy easy to grasp: Steve says "I believe a vegetarian diet is the healthiest diet". John replies "Vegetarianism is wrong, you know that Hitler was a vegetarian". Another example: "Stalin was an athiest. Stallin killed millions of people. Therefore athiest are evil".
This is a powerful and persuasive argument in that people do not want to be associated with those they do not like. The more disreputable the person being compared to is, the more psychologically powerful the argument. For this reason, it is important to be able to identify its use in order to avoid its influence.