You mention the word 'fan' - and 'fanatic' has come up in other answers, too - and 'fanboy'/'fangirl' would work well in many contexts. Obviously it wouldn't work everywhere - I can't see it being used with political parties, say. But its usage is quite broad - and ever broadening. It works well, obviously, for things like comics, technology, video games and other stereotypically 'geeky' subjects. Things which have cult followings are good, too. But it's also being increasingly used for fanatical interest in bands, TV series, book series and so on.
'Fanboy'/'fangirl' is, in my experience, almost exclusively used negatively by people who aren't fans of the thing in question. It is used to imply that a person's interest in the subject/product/series/etc. is excessive and unreasonable, and also that their opinions (and particularly defences) of it are ludicrously unobjective and disconnected from reality. It also implies that the person is likely to be difficult to argue with on the subject. However, it may be worn as a badge of honour - and taken up with pride - by the fan themselves.