Legal issues for a “fangame” of a commercial franchise?
For example, some of you may have seen Portal: The Flash Version. I was wondering what the legal background on this kind of stuff is, for example what legal things I'm supposed to say, give credit to the original's creators and all.
Obviously, I'm presuming that I'm not allowed to make money off of it, but I just want to know what restrictions there are of making a free Flash version.
For example, would I be allowed to make a Yu-Gi-Oh! game based off the Power Of Chaos series? And if so, would I be allowed to use the Yu-Gi-Oh name or would I have to call it something else?