"A "public suffix" is one under which Internet users can directly register names. Some examples of public suffixes are .com, .co.uk and pvt.k12.wy.us. The Public Suffix List is a list of all known public suffixes. "
I can't believe I didn't bookrmark this before. In any case, it's relaunched with a bunch of new features.
I keep thinking I should find a way to use this, and I keep failing to actually figure out a way to use this.