I'd been making attempts at this online game since late 1999, but I finally completed it this week. Not completed as in, got all the answers right...or even a fraction of them right...just "completed" it as in, stuck with it, logging in every week for 12 weeks, until it ended. My reward was finally getting all the solutions to the puzzles as well as access to one of the most exclusive forums on the 'net, lol. Because your access expires after your thirteenth week.
Anyways,
here is Planetarium. It is an online puzzle/story with 12 installments. Some of the puzzles are riddles, some are math, some are wordplay, some are other things. It's Really Really Hard to get some of them, those some depending on the person, but its pretty much guaranteed that no matter who you are, SOME of them will be hard. You have to log in at least once every week/week and a half or your username expires and you have to start over.