Escape from a nightmarish failure to contain cutting-edge science, of which you're partially responsible, and you've got one of the all-time classic game experiences, except it looks and plays cutting-edge. Half-Life enjoys near immortality in this homage-becomes-definitive community project that defies every business model of how a AAA title should emerge, virtually evolving into reality from a shared experience. I'm not sure how much more anyone can enjoy the genesis of 20+ years of quality video games than this.

Update: From the perspective of someone who played Half-Life way back in 1999, and thoroughly enjoyed the complete package it brought to video gaming, which also incidentally made me a PC gamer, the Black Mesa MOD was truly an amazing gift to that original community from the Crowbar Collective. After being majorly impressed they even pulled off that feat, I was blown away that Valve itself decided the MOD team should gain the benefits of selling it as an early-access game, with the future release of Xen (as well as immediately available deathmatch) being incentive to purchase it, which I did. Then the project really picked up steam (so to speak), and I have to say yet again how amazing it is that we've received Xen as well as the Definitive Edition updates, all within the span of 1 year. A more epic stepping off point to the rest of everything Half-Life (just think about all that) I can't imagine