This year, many U.S. voters are telling pollsters and other news sources that they intend to cast a write-in vote for President in November. The Richmond Times-Dispatch has this story, explaining to Virginia voters that only presidential write-ins cast for a declared write-in candidate will be tallied. Virginia is typical in that regard.
A list or table on laws nationwide might be even more useful this year than it “typically” would be.
(It might also help to know where and how often even legal and valid write-in votes aren’t counted, or tallied, anyway.)