is the concept for Tetris in the public domain or is there someone who is still collecting royalties on the concept? I mean, if I were to write a variant of Tetris would I owe somebody some money? Thanks.
The game, name, and all associated works of Tetris are owned and trademarked by The Tetris Company.
The concept of tetrominos, however, are not owned by anyone. Though The Tetris Company's website indicates the term 'Tetriminos' is in fact trademarked.
Whether you can create a clone of Tetris under another name is certainly up for debate. Recently a 'Tetris-clone' named Blockles was removed after litigation from the Tetris Company was brought against it. I have not been able to find the game in question to see just how close to the original Tetris game it was.
Tetris has been programmed in 1984 and sold commercially ever since. Since his author is not dead yet (I think), and certainly not dead for 70 years, it's definitely not in the public domain.
It's not an abandon-ware either because there's at least one company that claims to have the rights and wants to make money out of it. Now, as to who really owns the rights, that's highly questionable, as you can see in the Wikipedia article.