This is a follow up discussion from another post that was started based on some scare advertising from an attorney looking for clients.
Article on Gamasutra:
"Mona Ibrahim is a Trademark, Entertainment & Media law attorney based in Seattle, WA. She is Of Counsel with Imua Legal Advisors and her practice emphasizes copyright and trademark dispute resolution, IP registration, entertainment & media transactions, general business transactions and employment law. Mona is an avid gamer and is dedicated to serving the gaming and game development communities by providing education, helpful strategy, and legal assistance when necessary."
US Copyright Laws from US Library of Congress
Fair Use Law
Fair use is a US legal doctrine that permits limited use of copyrighted material without acquiring permission from the rights holders. ... Examples of fair use in United States copyright law include commentary, search engines, criticism, parody, news reporting, research, and scholarship.
My personal opinion:
I think it is important to understand the difference in a Fan Game, Clone, and Parody game.
A true Fan Game is a game designed by someone that is a fan of some form of media and usually follows a story with certain character features and would be recognized by those features.
For example creating a game based loosely on the idea of a group of characters fighting off hordes of Zombies and using unique sprites, scenes and music may resemble a well known TV series but you can find hundreds of those Zombie fan games all over the internet without legal challenges because Zombies was a common theme long before that TV series existed.
Clone games usually follow a specific set of movements, characters, elements and play associated with a well known game and Tetris is an example of a unique style of play that has been cloned by many game designers and has run in to legal challenges.
Parody is a bit trickier because you can under fair use laws in the US use a limited amount of copyrighted materials but the game must parody "an imitation of the style of a particular writer, artist, or genre with deliberate exaggeration for comic effect."
I personally think creating fan games and even clone games and parody games is a great way for developing game designers to practice their game design skills and trying to match the mechanics and game play of top games in your own game designs will teach you valuable design skills.
What I don't recommend is selling or publishing those fan clone games as once you start taking money for a game you are at much higher risk of having legal action taken by some big game company with itchy lawyers.
Nothing prevents someone from claiming copyright infringement even when no infringement exists and some game companies (Nintentdo) are more prone to send out DMCA notices to claim copyright infringement simply because they want to protect their right to legal action and ignoring fan and clone games would be relinquishing their copyright protection.