DeadComposers.net announces the last death anniversaries of composers before their works enter the public domain.
International copyright law protects the works of artists and composers for a certain amount of time after their deaths. When that time has passed, they automatically enter the public domain (on the next first of January), which renders them available for use by anyone without the need to worry about licensing.
While the question whether a certain work is in the public domain or not has to be answered for each work individually, the dates published by DeadComposers.net should provide a useful hint as to when it might make sense to check the legal status of a composer's work.