There's a useful thread going on over at Reddit on media conventions--are they useful for writers, how do you categorize them, ROI--things a writer has to consider. This is also useful for fans--where will you find instant entertainment, and maybe people who enjoy similar things? For me, also meeting people who enjoy different things is important.
This is a very male geeky list, and does not include conventions like World Fantasy Con and Sirens, very different conventions.