Public Suffix List
The Public Suffix List (PSL) is an attempt to build a database of Top-Level Domains (TLDs) including the respective registry's policies on domain registrations at different levels.
While the Public Suffix List has no opinion on alternative root systems, the list recognizes as authoritative and complies with ICP-3: A Unique, Authoritative Root for the DNS (ICANN, September 2001), allowing TLDs under the ICANN Root as well as inclusive sub-domain entries, and cookie apexes that comply with ICP-3 in a non-discriminatory manner.
Entries addition/updates/deletions are put through a loose process of vetting, curation, and validation by the community volunteers who help maintain the list.
Previously, browsers used an algorithm which basically only denied setting wide-ranging cookies for top-level domains with no dots (e.g. com or org). However, this did not work for top-level domains where only third-level registrations are allowed (e.g. co.uk). In these cases, websites could set a cookie for co.uk which will be passed onto every website registered under co.uk.
Clearly, this was a security risk as it allowed websites other than the one setting the cookie to read it, and therefore potentially extract sensitive information.
Since there is no algorithmic method of finding the highest level at which a domain may be registered for a particular top-level domain (the policies differ with each registry), the only method is to create a list of all top-level domains and the level at which domains can be registered. This is the aim of the effective TLD list.
As well as being used to prevent cookies from being set where they shouldn't be, the list can also potentially be used for other applications where the registry controlled and privately controlled parts of a domain name need to be known, for example when grouping by top-level domains.
The website for the Public Suffix List is at http://publicsuffix.org/.
Maintaining an up-to-date list of all top-level domains and policies is clearly a vast task, and therefore each registry has been asked to maintain their own section of the database and email any changes to the effective TLD list maintenance team, who will then merge it with the master database.
Outreach to Registries
Some outreach has been performed to help expand the awareness of the Public Suffix List within the ICANN Community - to bridge the divide between registries and the PSL community. Here are some presentations that have been made to the ccTLD community and to gTLDs at ICANN meetings since 2011.
- ccTLD Tech Day, June 2011 - ICANN #41, Singapore 
- ccTLD Tech Day, November 2013 - ICANN #48, Buenos Aires, Argentina 
- Current Effective TLD List (use http://publicsuffix.org/ instead)
- Wikipedia: List of Internet top-level domains
- IANA Root Zone Database
Mozilla Bug Reports
- Bug 9422 - Unsafe handling of illegal cookie domain attributes
- Bug 252342 - fix cookie domain checks to not allow .co.uk
- Bug 342314 - Need effective-TLD file