has anyone played the HL2 mod "causality effect" ? I got 3/4 endings, and for the half- life of me i can't figure what to do, to get the 4th ending. 

there is another mod, "DoomEd" has nothing to do with Doom, kind of on the lame side, but you will come out smarter after studying the periodic table section of the game, to pass the quiz and get into the storage room for needed supplies.