Action: Half life is an old goldsource mod where players played as movie characters in a deathmatch situation. Apparently the mapping scene became obsessed with intricate Easter eggs, until one deranged man known as Hondo created ahl_5am. ahl_5am is a simple, well-made deathmatch map, with a campaign of an easter egg that, for years, has gone unsolved. I propose that we all try a playthrough of it (one hint is that it requires multiple people to finish) to see where we can get. You can download action: half life here and the map here (along with some more info on the map). I plan to host an A:HL server for anyone willing to participate this Saturday, whenever the most people are online.
EDIT: just a note, there are some puzzles that will seriously hurt your head, so we will likely end up taking some breaks.