Honorary degrees

What are the generally accepted, legitimate fields in which honorary doctorates are awarded by academic institutions?

I know of no generally accepted rules for this. Each institution will make it’s own rules, but aren’t these inherently illegitimate? I mean, they’re specifically for people who aren’t following the degree requirements and can be tailored to the individual receiving the award. I’m sure that some universities are more rigorous about granting honorary doctorates (e.g., they want the person to have performed feats of academic rigor, or to have contributed greatly to society), while others grant the honor to large donors.