Users rate how good a job moderators are doing. The most famous site with such a feature is Slashdot. Users can enter "meta moderation mode" (or something similar) and will be shown a post that was moderated in its original form, and the nature of the moderation. The user then decides if the moderator acted fairly or not, and if not, in what respect the moderator was wrong. This information is used to improve the quality of moderation on the web site.
Slashdot and Kuro5hin are two examples of community with meta moderation enabled. The GameFAQs message boards formerly had a meta-moderation feature.