Official ODP Guidelines: Cool Sites - Definition: "A site that stands out among the other sites in the category or is the most definitive site on a subject can be marked as a 'Cool Site'. Ideally, since the cool site is the best site in the category, there should only be one. However, two cool sites may be appropriate in some circumstances. It is contrary to ODP policy for editors to cool their own sites or any sites with which they are affiliated. Doing so is grounds for removal."
Open Directory Cooling Log - Live data recorded as sites are cooled starting from June 23, 2000 (the date the feature was added).
Test/Meta/Self_Cooled_Sites - Archived historical record of why it's such a bad idea to ignore the guidelines and cool your own site.
Unofficial ODP FAQs - Covers the "which", the "how", the "how many", and what that "Reason" field is for.
Voiceroy on Cool Sites - Detailed FAQs relating to Arts/Animation/Voice_Actors but also offers general explanations as they relate to the rest of the directory. [See 3.14-3.18]
Open Directory Cool Sites - Archive of sites designated *Cool* by dmoz/ODP editors. Discontinued after "Cooling Log" feature was introduced. (July 6, 2000)