The Public Suffix List is a cross-vendor initiative to provide an accurate list of domain name suffixes, maintained by the hard work of Mozilla volunteers and by submissions from registries, to whom we are very grateful.
The usefulness of this can be seen if we take the example of cookies. In the past, browsers used an algorithm which 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 would be passed onto every website registered under co.uk.
Since there was and remains 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. This is the aim of the Public Suffix List.
Software using the Public Suffix List will be able to determine where cookies may and may not be set, protecting the user from being tracked across sites.
As well as this, the Public Suffix List can also be used to support features such as site grouping in browsers. By knowing where the user-controlled section of the domain name begins and ends, browsers can group cookies and history entries by site in a way that couldn't easily be done before.
For more information, please contact us.
These are some of the uses of the list we know about. If you are using it for something else, you are encouraged to tell us, because it helps us to assess the potential impact of changes.
In the future it may be used for, for example, restricting DOM Storage allowances on a per-domain basis.
QT uses it to restrict cookie setting from version 4.7.2 onwards.
WhoisMind uses it to get the domain name out of inputted URLs.
C, Perl and PHP: regdom-libs includes libraries for working with the Public Suffix List.
Java: regdom-libs has a Java port.
Python: publicsuffix is a Python module which gives access to the PSL.
Python: dnspy is another Python module which gives access to the PSL, and claims to be more flexible.
C: Faup, a command line tool with a C library and Python bindings
There's a list of libraries in various languages in the comments on this Stack Overflow question.