I am in the middle of creating a online game that was full of references to scifi books, movies, and television series. Can I release the game for free and people pay to use my server? They could download the game and play it off line, but to connect with other people you would need to pay a monthly fee. Is this legal? If not, what exactly constitutes a parody?

Parody is one of those hard to define, "I know it when I see it" sort of deals.

It really depends what you mean by a reference. If you have characters named Captain Kirk and Spock, you''re probably out of line. If you just have this guy that looks a LOT like Shatner, and maybe says one of his lines, I doubt anyone can or will do anything about it.