You're not dreaming. Your eyes do not deceive you. The legendary mod project Black Mesa Source - an almost complete rebuild of the original Half-Life - is now out and ready to go. It covers the whole game up to Xen, with that bit due to arrive later. Everything else is ready to go though, from the tentacles of Blast Silo to the headcrab zombies, and you can attempt to get at its files right now!
Congrats to the team for their accomplishment. As well as being available on its own, it's worth noting that Valve approves enough of the project to have made it one of the first successful Greenlight hopefuls - giving it a place on Steam along with the original game it's based on, and the far less ambitious update Valve did as a bonus for Half-Life 2 players. Work will hopefully begin on Black Mesa Source Source 2 once the anticipated next version engine lands. Or, for a real challenge, Daikatana Source.