From Wikipedia (h/t Wirkman Virkkala):
In 1974, Parker Brothers sued Anspach over the use of the “Monopoly” name, claiming trademark infringement. While preparing his legal defense, Anspach became aware of Monopoly’s history prior to Charles Darrow‘s sale of the game to Parker in 1935, and how it had evolved from Elizabeth Magie‘s original Landlord’s Game into the version Darrow appropriated. Anspach based his defense on the grounds that the game itself existed in effectively the public domain before Parker purchased it, and therefore Parker’s trademark claim on it should be nullified. The case dragged on for ten years, with numerous appeals and overturned judicial verdicts, until Anspach and Parker ultimately reached a settlement, permitting him to continue using the name Anti-Monopoly and distributing the game.
The game is currently in print, and is produced and distributed worldwide by University Games.