Half-Life 2 Episode 3 is certainly, with Black Mesa mod, one of the most speculating game on the web. Valve, who developed Alien Swarm seems to have put Episode 3 clues in the game's SDK!!

FPStime via www.fpstime.com »

It's pretty clear at this point: Steam is coming to Mac OS X. Valve sent out a number of suggestive images to large Mac and gaming sites today, that show a variety of Valve game properties done in unmistakable "Apple style"

primesuspect via gaming.icrontic.com »